Hello and welcome.
If it does not startup after connecting it to AC power and pressing the power button, it is probably a hardware issue caused by an electrical surge during the thunderstorm. Either the power supply, logic board, and/or other components were damaged. Because it isn't officially a user-serviceable Mac, we don't recommend attempting to open it up and troubleshoot it without the proper training/experience.
If it was indeed damaged, the repair parts alone could be more than half of purchasing a new iMac.
C