I also would like to help but you have not provided enough information.
Here are some points:
1. it could be a bent pin
2. heatsink is unlikely (should start up and then crash if not installed correctly) but maybe a possibility if the protection circuit detects overheating quickly
3. install ONLY the parts needed to start your system.
4. If you have spare parts (different video card) you can try substituting
I'm not really sure what you mean though. Reinstall all your hardware, make sure your processor has the correct power connector to the motherboard etc.
Usually when a system won't power up there's a motherboard issue or someone forgets the CPU power cable.
Think you could give us more details? Full system specs? Did you recently upgrade a component or is it a new build? We don't have much to work with here.
sorry ref stupidly description but had to rush out. when just the 24 pin mainboard power cable connected the fans spin and mainboard getting power cos the led on mainboard lights up green. when i connect the 4 pin cpu power cable i just get green light. no fans. its an asus m2a-vm hdmi. had done some work on it recently. is prob a bent pin or similar. end of day new mainboard not massive expense but i like to exhaust most possibilitys