So that I understand things, may I recap?
You have an iMac in sleep mode. Sudden loss of power to house, quickly restored. All else okay, but iMac won't start?
Starting with the simple: have you checked the fuse on the electric plug that goes into the wall socket? It could have blown, perhaps when the power was suddenly restored.
Or is your electric plug connected via an extension hub, and does that have a fuse?
In my, thankfully, limited experience of power loss, it is most unusual for the iMac not to restart. Loss of data or its corruption can occur, but even that is unusual when in sleep mode.
Please post back.
Ian