So my family’s PC was randomly shutting down and I determined it was because the cpu was overheating because the heatsink was too dirty. We cleaned it and all was fine. I decided to reformat it and install a newer OS on it, and when I was auto updating the display driver, it turned off. I thought maybe it was still overheating, so I opened it up and saw that the heatsink was pulsing on and off and I knew it was dying.
So I ordered a new heatsink and now when I try to turn it on, everything boots up perfectly but I get no display from the monitor, not even the POST from the BIOS. Do you think it could be a fried cpu or a mobo problem?
Any ideas?
try a reset cmos, could of corrupted it in the process. If no luck re-seat, cpu, ram and graphics card.
tried all of that and it didnt help 
The power supply must be your problem then. probably the reason why your system was randomly shutting down. some power supplies still power on the fans but don’t have enough juice to post the bios and boot windows. So I would definatly try another PSU.
but this pc was built and running for 7 months before anything happened to it…
That doesn’t mean anythin, any hardware can fail at any time. Especially if your PSU is cheap and underpowered for your system. That can stress the PSU and cause it to fail alot quicker.
But the main reasons why it wont post are because there could be a fault with the following:
Motherboard, CPU, Ram, Graphics Card, Power supply Unit and in some cases CPU fan failure or malfunction. You will just have to work your way through that list, starting generally with Ram, Power supply unit etc.