Motherboard stopped working

Under warranty they usually send out a new board. I guess it's cheaper and easier
than the labor cost. From what I gather the only time they send back the same board
is if the issue was BIOS related where they flash it and send it back. This is based
on forum users experiences, a different serial number usually comes back.

Shorting the power sense pin to ground could potentially fry components (other than
the board) if there is a major issue with the board itself. If you have tried everything
you can try that but be sure to remove all components from the board first. While
you have the board force powered check for any excess heat that could indicate
a dead board. I do not recommend trying with your components installed. You could
get lucky and a forced power up may clear power safety checks (with no components
installed).

You can also fire a support ticket off to ASRock:
https://event.asrock.com/tsd.asp

They may have further advice for you.

Unplug the system from power and leave it disconnected for an hour or so and see
if that brings it back.

If not, take the system apart, remove all components (including CPU) from the
board then make sure it's all clean before reinstalling just the RAM, CPU and
GPU. You can also try powering on without the GPU installed, even if you don't
have an iGPU. A dead GPU could also cause the power safeties to engage on the board.

If that doesn't work, try with another PSU, even if it's reading the correct voltages
on a multimeter, it may be unstable under load.

If after all that it still won't work then it's likely the board has died.