You do not tell us the age of which model of water heater you have, so all I can offer is some general advice.
Read the instructions for the appliance, and the caravans control panel. There may be some protection devices that may have tripped that need to be manually reset.
Check the water heater is full of water, by turning on a hot tap. Keep the tap old until you get a steady flow of water. That ensures the heater is full.
For gas operation, (This usually require 12vdc power to be turned on to run their control circuitry)
Look for any gas isolating taps in the gas pipe work to the heater. Make sure the one for the heater is turned on, generally the gas is on when the isolators handle aligns with the pipe feeding the appliance.
Check that any external clip on transit covers on the appliance's external flue have been removed.
Try restarting the appliance in accordance with the appliance manufacturer's instructions. If the gas bottle has been changed or the system has been inactive for a number of days, you may need to try and restart the applince several times to bleed any air that might be in the gas supply pipe work.
For mains electrical water heating, there may be a mains isolating switch associated with the appliance that needs to be turned on again refering to the owners manuals should tell you about any safety trips that may need resetting.