Whenever I have that happen on a PC, I turn off the power switch, unplug everything, and then press and hold the power button for at least 30 seconds to clear any residual power from the board. I then reconnect only the power and the monitor cable, turn the switch back on, then press the power button to turn it on.

If that doesn’t work, I’ll remove all the expansion cards, disconnect all drives, remove all the memory and then reseat only one memory board, and then try to boot it again.

If at that point it doesn’t work, last step is to reseat the processor. If it doesn’t work after reseating the processor, then you’ll need to test the power supply. If the power supply is good, then it’s a processor or motherboard issue, and I generally recommend replacing both at once if you have a failure of either one (my recommendation, doesn’t mean that others agree).