Open a terminal and type: lshw followed by pressing ENTER. Scroll back and
look for the section on display.

If it says Unclaimed, it means that you are not using the right graphics
driver. If this is the case, go to System, Administration, Hardware Drivers
and it will tell you if a hardware driver is available for your card. If so,
install it.

If it does not say Unclaimed, then post what it says to us and someone will
try to make sense of it.