It sounds like the onboard graphics have been turned off. Disconnect the mains from the PC, open the side and you will see a small battery and some jumper pins next to it. Remove the CMOS battery and then take the jumper pin out and put it onto the next pin along. Now put the jumper back to the original and then replace the CMOS battery.
Plug the mains in and put the monitor cable back into the onboard graphics