When the power goes off the canister won't pump dry - the water that is in there will remain so. I wouldn't be worried about the media and bacteria in the canister, it will still be viable after six hours. My suggestion, if you are home, is to stir the tank water around a couple of times every hour for about 5 mins. It will allow for gas exchange.
Your other alternative is to purchase a battery powered air pump - specialist shops stock them.