hardware

If LCD with multiple inputs it may have accidentally been switched to the wrong input.

Or if you are using a video card for your display output from the PC and your motherboard has onboard video the BIOS may have gotten switched over to the onboard video for output instead of the video card.

I don't think powering on the monitor will power on the system. (It wasn't clear if you were starting the computer or not from your statement).

If the computer is not started, that is exactly the type of thing a powered monitor will display. Have you checked the power cord for the computer to make sure it is plugged in? There also may be a failure in the computer's power supply unit if the cord is connected, the outlet is providing power, and the computer is switched on.