Did you check the circuit breaker? If you live in the US, did you flip a switch recently & thought it did nothing?

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).

i cant rly say to much without more context, but either send it to repair, or if u understand a bit, take it apart and put it back. take a good look at the components while doing that

If you take the side panel off can you see any lights on the motherboard that would indicate that there is power getting to the motherboard from the PSU.