Mac Power surge

This is Apple’s method.

If your Mac doesn't turn on – Apple Support (UK)

This is from an internet search.

“1) Unplug the iMac power cable from the back of the machine (not from the wall or power strip outlet)

2) Push in and hold the iMac power button for five seconds

3) Release the iMac power button

4) Plug the iMac power cable back into the back of the machine

5) Press the iMac power button normally. The machine will now startup

6) Be happy and breath a sigh of relief! “