Make sure the water heater is NOT in bypass mode.
Make sure the water heater tank is full of water.
Determine whether it's actually lighting, or is it just trying to light ? Go outside and stand next to it. Have someone inside turn the button on. Outside, you should hear the igniter click several times, then if it lights, you hear the whoosh/low roar of the burner.
If it lights, it will run until the water is hot. Then it will shut off, and only come on again when the water cools off.
Your water heater may also work on 120V electric, in addition to, or separate from the propane.
Again, make sure the tank is full of water. Do NOT attempt to heat it, either propane or electric, empty of water.