[ClockWork]

I've seen this happen in Leopard.

There are two possible cures:

(A) Shut it down. Start it up again.

(B) Reset the SMC (System Management Controller).

1. From the Apple menu, choose Shut Down.
2. Unplug all cables from the computer, including the power cord and any display cables.
3. Wait 30 seconds.
4. Plug the power cord back in, making sure the power button is not being pressed at the time. Then reconnect your keyboard and mouse to the computer.
5. Press the power button on the back to start up your computer.