There are two likely problems that can cause this. Both are fairly simple to resolve. Firstly, the cables connecting the monitor to the main computer base unit may have worked loose (especially if you move the base unit or monitor around much). The second is if you have any large magnetic fields near the monitor.
1. There are likely to be 2 cables leaving the back of the monitor. The first travels to the mains (or a power block similar to a laptop if it's an LCD). The other cable should connect to the base unit. Check both ends are firmly in place (but don't force it) and if there are thumb screws on the base unit, screw into place (the other end is likely to be a moulded plug anyway. If there seems to be resistance when you push it in, pull out the plug and check none of the pins are bent (turn off before doing this). These can be carefully straightened if required.
If that hasn't solved the problem
2. Move any electrical items such as speakers (especially hi-fi), mobile phones, cordless phones and wireless broadband boxes away from the monitor. Speakers are the most likely candidate.
I'd expect the most likely cause is number one from experience.
Hope that fixes it.
Loose connections on machines at home, work and on others' machines.