First thing to check is if the monitor cable has been disturbed; next try powering-down (everything) and leave it for a few minutes, power back-up and see if it fixes itself, if not Power-down, wait a few seconds, Power-up then press Shift & F8 as soon as Windows starts loading (you have to be quick, so might need several attempts), that should get you to the recovery Screen, then Try TroubleShoot and Automatic Repair.
If that fails post back with some form of machine spec.