Reset the SMC, as per > Resetting the System Management Controller
1. Shut down the computer.
2. Unplug the computer's power cord.
3. Wait fifteen seconds.
4. Attach the computer's power cord.
5. Wait five seconds, then press the power button to turn on the computer.
If that does not resolve the problem..?
Run the Extended Hardware Test, as per > Using Apple Hardware Test
If that does not work or you encounter a hardware problem, contact your local Apple Store or AASP using > Apple - Find Locations and have the Mac Mini checked out.