Resolution changed and can't be returned to original resolution.

[Tony K.]

I run several computers on a KVM switch.  I started up one of my
computers without switching the KVM to it first.  It booted up, but
the resolution was reset to 600x480, 60 Hz.  It was originally
1024x768, 85 Hz.  I tried rebooting but found the default is now set
at 600x480, 60 Hz and there are no other options in my System >
Preferences > Screen Resolution.

How can I reset my resolution options?

Ubuntu: Dapper 6.06