Help I've turned water of at the mains turned it back on and can't get any water to flow from hot taps

From you post I must assume that you do have cold water flowing. You should not shut off your main unless you first shut off your water heater. Too much pressure can build up from water expansion. Let's start by checking the supply line to the water heater and the heaters operation. Insure your fuel is on and that the pipes on the top are very warm. Open the drain at the bottom of the tank and chcek to see if there is a good force of water and that it is hot. Come back with your observations and we'll go from there. I'm sure others will jump in by then.