If you just got a black screen when using the System Preferences where you selected the new resolution, you may be able to select the previous resolution by using the up or down key.
If this doesn't work or you're not using the System Preferences, you will have to delete the file where MacOS X stores the last used resolution.
The file named Library/Preferences/com.apple.windowserver.plist contains the resolution that the System tries to use at startup. Deleting this file helps generally.
If this solves the problem during the reboot, but not in your user account, a similar file named "com.apple.windowserver.XXXXX.plist" exists in your own /Users/yourname/Library/Preferences/ByHost folder. You can remove this file to get back the standard resolution in your user account.