No-Signal on Boot

Hi, I'm currently having a problem. After I have installed my new heatsink my screen won't show on boot (No-signal/Enter power saving mode).

1. Remove Stock Intel Heatsink
2. Remove GPU (It obstruct my view)
3. Install my new heatsink
4. Re-install my GPU

After that, I start the PC and screen show no-signal (Enter power saving mode) and things I have tried after that is.

1. Remove and clean GPU contact with an eraser then put it back in.
2. Spotted that one of the RAM is loose. So I put it back in.
3. Remove and clean RAM contact with an eraser then put it back in.
4. Try to discharge current by pressing power button for a minute.
5. Remove GPU again to reset CMOS be removing its battery for about 30 min then put the battery and GPU back in.

But my PC still won't turn on. Can anyone suggest me how do I resolve this problem? Thanks!