Try clearing the CMOS/BIOS. Remove the video card first, and then clear the CMOS/BIOS. If you have any special settings you need in the BIOS, like SATA set to RAID, be careful to reset those before you allow the PC to boot. When you clear the BIOS you should be prompted to go into the BIOS or press a key to use default settings. The video setting should default to the onboard video if the video card is removed and the BIOS is cleared.
Nothing was changed on the PC before this happened? Do you have Windows update set to automatically apply updates? If so, a bad driver might have been installed.