These heaters have what's called a "sail switch," which basically gates the flow of propane to the burner. What you don't want is a situation in which propane is burning, but the fan quits. The switch is set right in front of the fan and has a little sail-like arm that effectively blows over if the fan works. If the switch is blown over, the propane is ON. If the switch is not blown over, then it assumes no fan, and the propane won't flow.
Now, sail switches are a darned good idea, but the sail-like arm must necessarily move easily (because mere movement of air must be able to move it). Alas, this is also a fairly dusty environment, so sometimes the sail switch gets bits of dust in it and won't blow over.
Last time ours went dead, I ordered a new switch (not that expensive). However, in the meantime I took out the old one and gave it a good blow-off with compressed air. Put it back in, and all has been well ever since. Now we have the old switch still installed (three years), and the new one still in its sealed package.
Anyway, it kind of sounds to me like you've got the symptoms of sail-switchitis: Fan works, no flame. As others have pointed out, check to make sure that the propane is on and making it to the heater. If it is, then maybe it's time to clean the switch with the air compressor.