I have a Dell Precision T3500 and It worked great up until I took it apart for cleaning then put it back together, upon putting it back together, it wouldn't power on.
First test I did was using the built in feature on the back of the power supply. I just held the power supply button down and it ran the computer completely fine with the led a solid green and everything plugged in.
I learned about this feature from this video.
After that, I used the built in jump feature that clears the ram I believe when it the jumps the power on, learned about this from the online manual found here (page 2). It's just a little plastic piece you pull off of 2 pins. When off, the power button works, I can use it to it turn the computer on and off. As soon as I put the plastic piece back on the 2 pin jumper though, the power button no longer works. When I got the computer on, it didn't display any out of the ordinary led codes or beep codes, just the normal 2 beeps on startup. Something strange though is that the OS won't start, it fails every time, I don't know if the ram being cleared has anything to do with that.
Just need some guidance on where to go from here. I can buy a new control panel for the front power button for only 5 bucks on ebay but after using that NVRAM jumper, I don't know If I need a new one.