ok, there's several things that could have happened:
1. resolution was changed, and it is set to something your monitor doesn't support
2. drivers were lost, you will need to reinstall them
3. you are having some hardware issue, check the cords. you might also be having hardware problem with video card.
to figure out which is it, try booting in safe mode first ( restart computer, hit F8 repeatedly until you get a boot menu, select Safe Mode and hit enter.)
If your monitor gives you the same problem even in safe mode, then it's a hardware problem. If it works fine in safe mode, check that the correct drivers are installed (re-installing them should probably do the trick)
if you just need to change the resolution, do it the normal way, right click desktop, properties, settings, set the resolution you need, hit ok. That should reset the resolution for normal mode.