I recently helped a friend add a 750 Watt PSU and a new graphics card to his PC, after the parts were in I found that the PC would not POST. The PC would just hang in black screen no logo or anything unless a key on the keyboard was pressed, then the PC would start normally.
I have already tried
-clearing the cmos
-disabling "turn on by keyboard" in the bios
- redoing all of the wiring in the case