Hi my name is XXXXX XXXXX am here to answer all of your plumbing questions. After the question process is complete please hit accept, Thanks What is the model of your heater?
The problem you may have is that on the propane tank inside the tank is a valve and if it thinks you have a leak it will shut off and not allow the gas out. This happens all the time.
So what you need to do is turn off the tank and disconnect the gas valve then hook it back up and this time make sure that the water heater is in the off positions then slowly open the gas valve after you have done this turn the gas valve on the heater to the pilot and try to light it This should do the trick.
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Do you Push down the pilot button when you try to light the water heater?
And if so when you hold down the pilot button do you hear the gas coming out of the valve?
If you do hear the gas then you may need a new thermocouple.
Here is a pdf file to show you how to replace the thermocouple on a lp gas water heater.