If the computer never responds to the power button or always restarts when the power button is held down, the button may be defective. Replacing a power switch for a desktop is relatively simple.

Power Options settings change or reset on their own? Power Saving Switch is changed? If Windows keeps changing power plan & reseting it at boot, see this fix.

Troubleshooting computer power issues can be difficult, as the problem could originate with the power supply unit, or PSU, or the motherboard.