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.