Can you make sure /apps/compiz/general/screen0/options/detect_outputs is checked in gconf-editor? If you can't get to that easily boot in recovery mode (option in grub) and change your xorg.conf to use the 'vesa' driver instead of the 'mga' driver.
Can you make sure /apps/compiz/ general/ screen0/ options/ detect_ outputs is checked in gconf-editor? If you can't get to that easily boot in recovery mode (option in grub) and change your xorg.conf to use the 'vesa' driver instead of the 'mga' driver.