It could be that the PMU has become corrupt. Ypu ma need to reset it.
To reset the PMU on a Mac mini:
Unplug all cables from the computer, including the power cord.
Wait 10 seconds.
Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.
Let go of the power button.
Press the power button once more to start up your Mac mini.
Important: This procedure also resets the computer's PRAM. Be sure to reset your computer's time, date, and other system parameter settings as necessary.