You may have damaged your PCI slot or your gfx card. I had a PC (waaaay back) with a damaged AGP slot. The best fix was remove card, push against the board, replace card, boot. Obviously the Orcish approach. Sometimes just wiggling it worked. Was definitely the slot as my card worked in a friend's PC.... Back to the point...
To Diagnose:
1. Power off completely
2. Remove the video card.
3. Boot up (I'll assume you've a board with
some onboard graphics).
If you can boot fine in normal mode, then I guess your question now becomes "Card or board?" which you can solve by testing the card in another PC.
If (and only if) your boot fails, check the board's manual for the BIOS RESET PINS location and use either a jumper (or a piece of conductive metal if you're experienced and have a steady hand) to short the BIOS RESET PINS. The reason for this is some BIOS won't detect that your card isn't there and therefore not default to onboard video (Personal experience: don't buy cheap boards, lol).
If you are now able to boot your PC then you'll have to test your card in another PC or another card in yours (to see if it is in fact the PCI slot or the card).
The issue may resolve itself during the process (could be moisture, dust, being loose, etc). Also, it either is stuffed, don't beat yourself up, it may have coincidentally packed in. It happens.
Can't make any guarantees on this advice but it should diagnose your problem. I hope this is of use.