Could you try remounting the CPU heat sink. Also, try booting with only 1 stick of ram and one primary VGA (if Sli). Remove everything else, CD/DVD, HDD's, USB's, etc; Make sure you discharge the motherboard before starting, hold power button in while touching chassis for a few seconds.
Also, if the "heat sink" you are referring to is the CPU cooler, remember that every time you remove it, you need to clean off the old thermal compound from both the CPU and heat sink and apply a small amount of fresh thermal compound.
Can someone tell me why a poor connection between the CPU & heatsink causes a PC not to power up?
I'll have to get some thermal grease and replace the old stuff tomorrow when the shops are open - but I just wondered why it stops it powering up at all? I would have thought that maybe it would just run poorly.....
How do I clean off the old thermal paste with damaging things? It's dry especially around the edges. Far too much was put on originally and it's squeezed out into the fins! Could that have caused problems... ?