umm - there should have been a fail safe mechanism to prevent this very situation.
Normally (in mac OS X), just before you change the display settings the OS system automatically comes up with a pop-up window for which you have to press "OK" in order to proceed. It states:
"About to change the display mode...
If your display becomes black, wait 15 seconds or press the Return or Escape key to revert the display mode back to how it is now."
(For clarity - once the settings have changed another pop-up window appears asking you to confirm the new settings or it will revert in 15 seconds...)
So, provided you waited 15 seconds the OS should have automatically reverted to your original settings. But obviously this didn't happen. Since the display settings are determined by the OS and you have used a friends monitor to manually change the settings back... I fear you might have damaged the monitor.
However, I'm not too familiar with LCD monitors and their limitations. Maybe check around for a reset switch??? Maybe try your monitor on another computer???
Sorry I'm not too helpful... Anyone else???