Don't see how you remove & reinstall the mobo without also removing the graphics card (unless it's integrated of course).
Even so, whether you did or not, I would reseat the graphics card, they are notoriously twitchy when it comes to being securely inserted etc.
If that doesn't fix it, I would remove everything that isn't absolutely necessary from the system (ie all drives/cables, pci cards etc). It may also help if you remove the mobo from the case - it makes it easier to work on & eliminates any possibility of the case shorting out the board etc.
Could also try clearing the cmos to reset the bios.