I push the button, it doesn't start.

[thorn14]

I was doing a valve check and after I finished I was testing that it still ran/warming up for an oil change, it started and I changed the oil not touching a thing during the oil change.

After the change, I try to start the bike, fuel pump primes, needles sweep, nothing weird, I press the starter button and I get no click from the solenoid.

About the bike...
2002 Monster 620 with an 800 engine that has a reflashed ecu with the immobilizer disabled.
As a 2002, it doesn't have the clutch microswitch, so I have permanently disactivated it by shorting that pin (BODY 33 I think) to ground which has worked for months. I doublechecked that it's functioning.
I also have a kickstand bypass.

So what I have checked...
Killswitch and starter button work.
Solenoid clicks with external power.
Starter engages when given 12v.
The wire between the killswitch connector and solenoid connector has .03 ohm (resistance?)
The main fuse box is all good.
Battery reads 12v and while I haven't got it load tested, I'd assume it would at least try to engage the starter.

What I think needs to be checked
Wiring harness.
Kickstand bypass, I'm just not exactly sure what I should be looking for. I could try uninstalling the bypass.
Solenoid may not be functioning but I've been piecing together searches from around the internet and I think it's ok?

At least my valve clearances haven't changed since I installed the MBP collets. 
The harness for pinched/broken wires around the steering head. (I have a separate harness I was hoping to put off starting on.)

[ducpainter]

Remove and reinstall the small 2 pin connector at the solenoid.

They can easily be dislodged when checking valves.

[Howie]

What type of battery do you have?  12 volts on an AGM battery (OEM) is less than half charge, lithium close to dead.  If ducpainter's suggestion doesn't help charge and load test battery.

[NAKID]

Any possibility you loosened the starter cable while you were changing the oil?

[thorn14]

Charged the battery and got it load tested and it checks out ok at 13.1v.

Reinstalled the two pin connector and nothing.

The starter cable is intact and upon 12v power the starter spins.

I had begun following this advice...

Have someone push the starter button while you check the Red/blue wire coming to the starter solenoid.
12v yes? solenoid is bad or bad starter.
12v NO? proceed to follow the red blue wire to the connector, unplug the connector and find the pin coming from the red/ blue wire from the wiring side, which is pin D (not the pin going to the solenoid obviously) check for 12V when you hit the start button.
12V Yes? there is a bad connection between the connector, there is a short in the wire from the connector to the solenoid.
12V no? follow the red blue wire to the start switch, unplug the start switch connector
Ohm test the red/blue wire from the switch connector, Wiring side (pin 4) to the last connector (pin d) should have .05 ohms or smaller. if its higher , theres your burnt wire.
After ohm test, while the switch is unplugged keep the bike fully powered and test the switch connector red/black wire for volts (pin 1).
12V yes? switch is bad. clean it, take it apart, buy new etc.
12V no? Check Fuse 2.

I want to isolate whether it is a hardware problem, or an ECU problem.

[mgrisham]

Is the ground wire (large and black) disconnected from the ECU?

[thorn14]

Yes the black wire is connected.

After rechecking all my work, I believe the starter solenoid was in fact bad and I have one showing up Tuesday and will try that.

[thorn14]

So the solenoid is switched for a new unit and still no click.

I'm frustrated now, but need to move forward so...
Kickstand works. Clutch microswitch bypass works. Right side killswitch and starter button work.
My loom does have all the wires around the steering head all make the beast with two backsed up, but as far as I can tell, they seem to work.

I guess I am going to triple check all my work. I've got a spare wiring harness which is really looking like a much more enticing option. 

The one thing I can't check is the ECU. Would any diagnostic software be able to get anything?

[DarkMonster620]

Try the Centurion S for IAW59M, it is about $229 plus S/H

[OT]

For the oil change, did you set the bike up on a rear stand or other "stand" that might have pinched a wire somewhere (e.g., underneath the bike along the swing arm or near the engine)?

[thorn14]

It was up on a rear stand.

I'm guessing I pinched or finally severed one wire in the harness that is crucial to starting.

[Howie]

The answer is diagnostics.  The purchase and replacement of the solenoid was a result of no diagnosis.  You do not want to do the same with the harness.  The solenoid is a fairly simple part.  Energize a coil, plunger moves, contacts close, current flows, starter turns.  First step is see if it is getting power 

Have someone push the starter button while you check the Red/blue wire coming to the starter solenoid.

(quoting myself I think).  If power is there and the other wire is a good ground you would then want to test the solenoid itself.  If you  know what the resistance of the coil is you can measure it with an ohmmeter, if not, no problem since you are not looking to repair it.  The  decisive test is apply voltage to the coil, voltmeter positive on the connector from the battery, negative on the connector to the starter.  You should read less than .2 volts (voltage drop, voltage consumed by the component).  Back yard alternative; jump solenoid and see if the starter cranks.

Go back here and get to work, or maybe consider getting the bike to a qualified shop.  There are wires here

My loom does have all the wires around the steering head all make the beast with two backsed up, but as far as I can tell, they seem to work

This is very likely where your problem is.

[thorn14]

I was following the checklist I had for testing the solenoid and when I used my voltmeter to one of the wires leading to the solenoid I was getting  around 12.9V, so I determined the solenoid to be at fault according to step 1 of the checklist. I just went back and checked both wires at the connector leading to the solenoid this time and one wire is delivering 12.9V, but the other is showing around 12.5V.

The wire in question is Red/black from BODY pin 1 (O2HEAT2) and goes directly to the solenoid along with going to the diode.

Also, for fun, on the old solenoid, one post is completely loose, and the other wiggles a little, is this normal?

Edit: I get 12.9V across the two terminals for the solenoid, and jumping them cranks the starter. Also the wire has a resistance of 1.5 ohms, not the .1 ohms or less the checklist I have desires. Furthermore when I switched the connectors for the new harness, the wires had a little blackish corrosion on them.

[NAKID]

At any point have you disconnected the battery and/or ECU ground wire? I know I did once and hooked the ECU ground mistakenly to the negative post on the battery. It resulted in a similar situation you have now. Once I reconnected the ground wire to the proper post near the ECU all was good.

[thorn14]

I have disconnected the battery and ecu when I initially checked my valves but it ran after that. Since then I have disconnected my battery a few times.

Double checked and it is in the proper place.

One thing I thought of, for the solenoid, do I need to have the correct wires hooked up? The new solenoid came with a different connector and I wasn't sure which goes where.