Locate the sail switch and try some WD40 onit. Make sure it moves freely. Its purpose is to shut down your heater in case of blower failure. Lint and debris gets into the sail switch operation preventing it to work - very common problem...
Another possible failure point if you have propane making it to the heater would be the heater control board which runs off of 12VDC. This could be just the 12VDC has been shut diwn or drained batteries etc... Perhaps your battery disconnect switch has been engaged...
Roy Ken