- Is this an iMac's builtin display?
- Is this an external display used with a Mac-Mini?
- Is this an external display used with a Mac Pro?
- Is this an external Display used with an iMac?
More details always helps with helping us visualize the situation.
What have you done so far?:
- Have you tried adjusting things within the "Display" System Preference?
- Was the Univeral Access "zoom" feature turned on by accident?
- When you say you can only see 2/3rds of the screen...do you mean 1/3 of the display is "off in space somewhere" (outside the borders of the display)? Or is 1/3rd of the display actually black/blank?
- Nick