Have you tried doing a hard reset on the computer, if note, or you don't know how, unplug the PC for about 15 seconds and reconnect it and boot the computer up. Make sure the monitor is plugged to the tower before booting back up. The system might just need to be refreshed.
If that doesn't work, since we know that the monitor still works, you might need a second monitor to use so that you can go into the control panel to check the display drivers. But you probably won't need to go that far.