Hold down the power button for 30 seconds. This closes the circuit and ensures all power from components is drained to clear the software connections between the BIOS and hardware and clear any corruption in the temporary memory.
(If it is a laptop, plug the battery back into the laptop and then) Plug the computer back into the wall. Do not reconnect any unnecessary peripherals; monitor, keyboard, and mouse should suffice and be the only peripherals reconnected.
Turn it on to reinitialize the software connections between the BIOS and hardware.