Turn off PC...............Turn on PC, as soon as you hit the power button,
start hitting the F8 key, repeatedly and quickly........you should eventualy
get a dos type screen that you can select to boot into safe mode.
Once in safe mode, (if you know how to set your video resolution + refresh
rates) you're all set. Change them.
If you don't know, right click on the desktop, select properties, and look
around in there. Especialy the display or monitor tab. My guess is they
changed the refresh rates so high that your Monitor cant support it. Hence
the Black screen.
ALso, as silly as this may sound,,,,,,,,,before you do anything, make sure
that:
1) Your monitor is actualy plugged in.......TO A JACK THAT WORKS
2) Your monitor is actualy plugged into your PC.........CORRECTLY.