If your hot water heater has been unused for a period of time in storage, the propane has leaked out of the supply line; and the line now contains air. The red light is a "failed to light" indicator. If the water heater is working, the red indicator should be OFF.
To purge the air in the propane line to the hot water heater, light the burners on the stove. This may take several attempts, too. When your stove burners stay lit, propane is most of the way down the line to your hot water heater. Also, if you haven't checked, your refrigerator burner may not be lit either. This will purge that line, too.
Turn the hot water heater switch OFF, and wait a few seconds for the system to reset. Then, turn it back on. You'll hear clicking (outside), and the burner should ignite. If it doesn't, after a short pause, it will attempt again, two or three times. Then, the red light will come back ON again.
If it does, just repeat the above procedure until the hot water heater burner successfully ignites.
Use this same procedure for the refrigerator, if it doesn't start. However, the hot water heater is down-line from the refrigerator; so if the water heater starts, you should have no problems with the refrigerator.
IMPORTANT NOTE: Make sure the propane tank is at least 1/3 full and the valve is open. I know that this sounds dumb, but I have personally forgotten to do this more than once.
The empty weight of a 30 pound propane tank is stamped on the metal handle, and a full tank weighs around 55 pounds. I use a cheap luggage scale to weigh the tanks when I suspect they need to be filled (around 1/4 full). The one below is about $8.00. You can buy a better one, but extreme accuracy isn't required.
Protege Luggage Scale and Tape Measure: Luggage : Walmart.com