I just tried to turn on my pc this morning and it does nothing but black screen. It doesn't even turn on the monitor. There are no beeps or anything. I noticed the fan on the processor would start then stop about 3 seconds into being powered up.

I was thinking something just decided to take a crap but I wanted to ask to be sure first. Sounds like the power supply.I just tried a secondary power supply that I have and still have the same exact results. I guarantee this power supply isn't bad. So it has to be something ELSE. I BELIEVE I have it narrowed down to either the motherboard or the processor. Try to reseat the RAM in the machine, and if no difference, reseat any addon cards. (Graphics, sound, modem etc)Or take out all addon cards and test pc with only Processor, heat sink, RAM. NOT even hard disk.
Also, take out RAM and SEE if you hear beeps?
If you do hear beeps, try with another/new cmos battery. Try removing all ad on cards, graphic cards and connect the monitor CABLE to the on BOARD graphics and start the machine. all instructions above are possible.. if you done with all that instructions and still and did not fix the problem, try to clean the power supply, heat sink, mobo, reset the RAM and place new thermal paste on the processor.