i just replaced the heatsink on my computer from the stock Athlon to the stock with black edition, (i know, a small upgrade but still better than before) but now it wont boot at all, not even to bios. the case light and all fans come on but thats about it. I built this about a year ago and have had no problems prior to this.
Did you loosen the cpu power connector that is near the cpu socket or any other power connector , because it does sound like that could be the problem. Is the heatsink the only thing that you replaced and you did not take the cpu out of its socket. Make sure that the ram is seated all the way in and you didnot loosen it as well. By replacing the heatsink there are a few things in the area of it that you could have accidently touched and loosened and didn't realize it.
when removing the old heatsink the cpu did pop out, since they use the same hardware i did not have to remove the motherboard or any other connections, if i remove the heatsink to re-seat the cpu will i need to replace the grease? its only been on for a day