If you've just de-winterized, make sure your water heater tank bypass valves are in the correct position. Once in the correct position, the tank needs to be filled with water. If hooked to city water, just open a hot water tap until water begins to come out. If using your onboard tank, turn on your water pump and then open a hot water tap. It will take a couple minutes to fill. If your tank is bypassed, you'll get water almost immediately because the water circuit is bypassing the tank. So if you're not sure of the status, you'll need to check the valves. You could also do a quick opening of the pressure relief valve on the back of the water heater, but if it IS full and you've heated it up, this could spray dangerous hot water so I don't recommend that unless you are positive that the water isn't hot.
You need to make sure the tank has filled before turning on any water heater elements/burners or risk burning them up.
Then follow advice as has already been given. When on gas, our tank takes about 20 minutes to heat up the water. A bit longer on electric. You can confirm if the gas burner is working by standing outside at the water heater access panel. The gas burner is relatively loud, you can't miss it. Electric is silent though, you'll just have to wait and see if water gets hot.