A couple of things I would check for,
(assuming you can see anything on screen at boot, BIOS type/name ? Yes/no?)
1) Check in the BIOS to make certain that the Video card is being selected as first choice.
2) Does the graphic card need an extra power connection, from the power supply, aside from the one in the edge connector?
3) If you used the same Motherboard & video card in the old case, was it ok then?
If yes, then check very carefully how you are bolting the motherboard to the case.
Check if you have enough standoff's under the board, as just leaving one out can cause problems. Normally you would have 4 or 6, but check your paperwork if you have any.