Monitor not recognised after sleep

Has anyone any ideas please, after starting up after standby, my monitor has no input signal this happens about once in every three times. Because there's no screen, I have to force quit!

Phil

There is probably some kind of hot key combination that will activate it.

I think there's more to it than that as there's no input to it, maybe a graphics card problem? I can't find a keyboard shortcut anyway to turn a monitor on, in fact the monitor is on anyway when this happens.