The gas is on? Can you hear it try to ignite? There is water in the water tank, right? Does it work on 120VAC?
If it tries but does not ignite, check to see if the ignition box is clean.
If you can't hear it, are your batteries up or low? The water heater needs 12V to ignite.
I ask this because the water heater does have an overtemp feature that will need to be reset if it over heated.
If it will run on 120VAC then it is a gas related only issue but could be low 12V.