Can you enter the BIOS? Even though you can't see the post screen, turn on the PC and try hitting f10 or whatever the key is for your MOBO to enter the BIOS. After you've *hopefully* entered the BIOS (even though you can't see) press esc, left arrow key, enter. If the computer restarts, the problem is display.
What was your old motherboard? Was it the same exact model and chipset of your current? If it was different... you'd likely benefit from reinstalling windows, but it shouldn't be causing you not to be able to see anything.