After thunderstorm my iMac will not work.

Not a good sign :-(

In most cases the system is to far gone to repair. You'll need to start with replacing the power supply, then on to the logic board by that time you are so close in the replacement cost of the system (used) it doesn't make economical sense to even try repairing.

As to protecting your next system. Think about getting a surge suppressor and plug in the computer and its peripherals. Don't forget the cable or modem too! If you can swing it I would get a UPS as that will offer more protection.

The other part here is the building wiring and grounding. You'll need to speak with an electrican to check your outlet wiring.