Possible causes;
1. A loose connection between your PC and your monitor. (Check that everything is secure).
2. A fault with the monitor. (Ideally try to borrow another monitor from someone, to see if the problem persists).
3. A corrupt video driver file. (Take a look in the Device Manager to see whether there are any exclamation marks shown. Even if there aren't, it might be worth locating, downloading and reinstalling the driver file).
4. A temporary software glitch. (Try turning both your PC and monitor off and on again!).
5. A hangover. (Stay off the hard stuff!).
My suspicion is that (unless it's just a loose or damaged cable) your monitor may well be about to pack up. The only way to find out is to try another monitor (or possibly to try your existing monitor with another computer, to see if the problem persists).