Even though the iMac wasn't turned on the power supply (which is external) likely got hit from the surges.
So as koppie007 stated you should visit the store where you got the system and make sure to bring the power unit as well. Don't offer any details of the power event just simply say the system won't turn on now. Let the tech discover the issue. Most likely they will just give you a new power unit and call it a day.
Now you do need to make some corrections to your power service! I would get an electrician in to install a building wide surge suppressor which is a very easy device to install at your building/office breaker panel. In addition you should install discrete power strips with a fused switch so you can quickly isolate your equipment (all sensitive gear).