I changed my heatsink fan and after I re-essembled it I found that it wouldn't boot. I've tried everything down to putting the old fan back. But nothing works! HELP

Which part of the booting process does it fail? Can you get into the BIOS?

What you described seems unusual. I've done it dozens of times, never had a problem.

It could be a matter of static discharge, during the removal. You won't feel it, but the cpu and/or motherboard will.

You can try the cpu in another working compatible system, to check that the cpu is still good.

If its good, then most likely static discharge damaged the motherboard. Other components may be damaged also, so you need to individually check them.

Don't worry about hard drives and cdroms, they rarely if ever get damaged due to static.

I changed my heatsink fan

Howdy and welcome to AMHD:

Did you apply some sort of thermal paste (Artic silver, etc) when you changed the heatsink/fan?

If not, odds are your CPU is overheating.

Murray