Video cable, GPU, or monitor. Test with a different computer, test with a different cable. If a different computer works, then it's the GPU. If the GPU's not integrated, probably worth replacing it, or at least using a different port if it has one. If it is integrated, it's on the motherboard and it's a generally bad sign when a motherboard starts failing, and you probably want to replace the computer in some form or fashion, though it's pretty unlikely that that's actually the case unless your computer is ancient and has poor cooling and lots of dust. If neither a different cable or a different tower works, it's probably the monitor.
(Obviously, it could also be a weird driver issue or something like that. If so, good luck.)