First, welcome to RVF,
@JMurray; glad to have you join us.
Does your gas water heater have a pilot light or a spark igniter? In that era, I think it could be either.
The most likely culprit is a failed thermocouple; the device that shuts off the gas supply if the flame goes out. Generally, they are relatively easy to replace. It is also possible that the electrically-operated (by the thermocouple) valve has failed. Again, usually not a major replacement issue.
I'm pretty sure you have already checked to be sure that the propane tank has gas in it, but that's certainly the first step. Is there gas available for the stovetop, furnace, etc.?
And, if by chance, you have just refilled the propane tank and that's when the problem occured, the internal tank safety valve may have tripped. The solution is to close the tank valve, wait a few minutes and s-l-o-w-l-y re-open the tank valve. That usually resets the safety valve. If, instead of a chassis.mounted propane tank, you have a portable one; closing the valve, disconnecting the supply hose, bumping the base of the tank on the ground a couple of times, re-installing the tank, reconnecting the supply hose, and re-opening the tank valve (again, s-l-o-w-l-y) should fix the problem.
TJ