The other problem is that once the bacteria in the filter dies off, you really need to wash out the media and start again. If the filter is off for a long period of time it can cause anerobic bacteria to produce nasty chemicals that can poison the tank when the filter starts again. A few hours is ok, but a prolonged period of time can be quite problematic. Trust your nose, if the tank smells bad when the filter starts again (a normal tank should smell like warm earth, a bit like a reptile house at the zoo), take it out, wash the filter out and do a large water change on the tank. I would do a large water change anyway to dilute the ammonia waste that would have built up from the fish. At this point you may go through a mini or perhaps more extensive cycle as the filter gets populated with bacteria again. Like Majjie says, if the tank is quite mature there will be colonies of nitrifying bacteria that will rapidly populate the filter again once the water begins to flow round the tank, but a newer tank may take longer to cycle again.
At this point you may well be cycling with fish unwittingly, so test the water daily, and if you see any ammonia or nitrite in the water, do a water change. Now this is a double edged sword because diluting the ammonia means the cycle takes longer, but is better on the fish. You could leave the water changes till the ammonia rises to a higher level but while the cycle wouldn't take as long, you could suffer more losses from your fish stock, particularly more sensitive fish.
Just a few pointers that are probably in a pinned topic somewhere, and have been mentioned in practical fishkeeping in their recent disaster management series.
