Assuming that you mean that you cannot load the GUI then:
Edit /etc/X11/xorg.conf or /etc/X11/XF86Config-4 depending on whether you're running XOrg or XFree86.
Look in Section "Monitor", which look like this:
Identifier "Generic Monitor"
DisplaySize 280 210
HorizSync 30.0 - 70.0
VertRefresh 50.0 - 100.0
Since You're editing outside of any wizards the important thing is to vary the HorizSync and VertRefresh parameters. You've probably set the upper limits too high.
If the error is on a file associated with the KVM switch please ignore this post!
You might want to connect a monitor directly to the computer to check out whether the config file needs changing.