Do you have an external monitor connected? Check the manual to see if there is a keyboard function combination to select display output device. Maybe some program you installed is switching output devices.
If you don't
have an external monitor in use at the moment, can you connect one? Then you'll have a picture to troubleshoot. If you don't have a hotkey on the keyboard for switching the primary display, you can try it through Windows' screen settings (if you can get a picture on an external monitor, that is).