Can you get into bios?

reset your bios and reboot, its worth a try.

btw if that power button does not turn off pc, just hold in on it for 5 - 10 seconds and it will shut down.

I would double check all cables/connections to make sure you didnt knock anything loose during the cleaning process. Are you getting any beeps or anything during boot?

If you completed all the suggestions posted including taking all components off the main board except one stick of ram and cpu then i would have to say its being caused by your tower ( that is if...

Your hdd could be fried. is this your os hdd? if so you could try plugging it into another pc and running checkdisc or another utility to check for bad sectors etc. You may be able to recover some...

If there are no beeps its not the video. You would get beeps, probably one long beep followed by two short (from my experience).
If there are no beeps but fan come on i would have to say remove...

yes that can happen! maybe just not enough power. that exact same thing happened to a computer i was fixing and when i tried another suppy, boom, up she came.

did you try hooking the pc to a different monitor to determine if that is the cause? and if so how about cables/connections? just a shot in the dark yes, but always start with the simplest possible...

Sounds to me like your spending money before you should. If you removed all the components attached to you board and didnt get any beeps i would definately try plugging in into a new power supply...