Comment 21 for bug 549741

Had the same problem, and found the answer online. Disconnect your laptop from your power supply, and then pop the battery out for a few seconds. Reinsert the battery and the power supply cord and boot up. This should take care of the problem for a while. I've found that the problem tends to reappear when the power supply cord is loose and you get a bootup message stating that the system can't recognize the type of power supply being used. If that happens, pull the power and the battery again.

Good luck!
Jeff