I'm going to go out on a limb and guess this is on a laptop. I'm also going to guess that you mean 'entire screen'.
If that is the case, I've seen this happen on certain models of some brands of laptops when you put it in 'Presentation Mode' without an external display hooked up. Basically it's because some of the built-in graphics drivers get a little confused when you change their profile settings.
Seek out your Fn and the key (usually F5 or F9) that is for switching to your external display. The symbol on it usually looks like two rounded rectangles.
Sometimes it takes a combination of this key sequence and closing/reopening your laptop screen to set things right.