Have you by any chance hooked the monitor up to another computer? How about hooking a different monitor up to your computer? If it's possible this is the quickest way to find out for sure what's wrong!

It sounds like the monitor is on its way out. Is this monitor an LCD or a CRT? If its a CRT it would make a good boat anchor (usually not fixable). If it's an LCD it can be fixed but it may cost more to fix it than a new monitor would cost.