I have an HP Compaq nx9010. The LCD display was broken and removed a few years ago, since which time I have been using it essentially as a desktop machine with an external monitor and keyboard/mouse. Recently I'd been having trouble with overheating shutdowns, so I disassembled it yesterday and cleaned the dust out of the heatsink fans (the air flow was almost entirely blocked). Up till then it had been working fine, albeit slowly.
Unfortunately, when I reassembled the machine it would not power on. I stripped it back down to the bare essentials and then it did, so I replaced the components individually and checked after each one to make sure it still worked. When I finally had it all back together I plugged in the external monitor only to discover that nothing would appear on the display (the monitor's power LED simply blinks as it does when it is connected to a computer that is not switched on).
When I attach the AC adaptor and press the power button the switchboard PCA LEDs light up (power, caps lock, num lock) and one of the fans on the heatsink spins up briefly (is this normal---should both fans spin up?). The hard drive seems to spin up but I don't hear it doing anything until it spins down on power off---the only sound is a slow, faint ticking from the CPU/heatsink area, which I am guessing may be due to thermal stresses on the material as it heats up. When I had the CD/DVD drive attached (I've since removed it) it made a regular clunking sound on power up, like it sometimes does when trying to read a corrupted CD (the drive was empty). There are no beeps. The main status LEDs are disconnected, so provide no information.
I've tried various configurations of RAM (I have two sticks, which I've tried separately in each slot), in between which I pressed the pinhole "reset" button on the bottom of the case---I suppose this resets the BIOS for a new hardware arrangement, or something (I'm not exactly sure, but presumably it doesn't hurt). Since the whole thing was working yesterday I'm fairly confident that most of the parts---e.g. AC adaptor, RAM, etc.---are in good condition. My only guess, from what I've read, is that either I've reassembled it incorrectly (and I've done my best to check that this isn't the case; I have dis-/reassembled this laptop before with no trouble) or that the motherboard or CPU has died somehow.
It could be that disassembling might have caused the thermal paste (old and dried-up anyways) to fail, therefore causing overheating of CPU (same could have happened with the GPU). If the design allows it, you might want to re-apply some thermal paste or compound and then reinstall the heatsink.
Thank you for your suggestion. That's something I will bear in mind, although I don't know that it could have caused the symptoms described above. In any case, I have it working again now. I don't know exactly what the problem was: I just took out the heatsink and CPU, looked at them a bit and then put them back in again---precisely as I had done before, or so I thought. I suppose there must have been some subtle change in placement somewhere.
That happened to me once. I pulled the processor back out and found I had bent one of the pins. I straightend the pin plugged it back in and unbelievably it worked then. suprised the pin didnt break when i straightend it!