If it didn't attempt to start at all, no power light on the switch panel, or "clicking" of the igniter (I'll assume that it is DSI), then, the most likely culprit will be an electrical fault of the breaker/fuse/broken wire variety.
If it attempts to fire up (you get the power light at the switch) and you hear a clicking of the igniter, then the most likely cause is a gas supply interruption. Gas supply problems could be insects in the burner tube (spiders love the smell of propane as much as mud-daubers love building their tubes in the burner pipe). Gas supply problems could be the insects or a bad themister (the device that detects a flame). If, for any reason, the water heater attempts to start and there is no detection of a flame, it will kill the supply of gas to the regulator...that would be in the "electrical" problems, but, the water would attempt to start and would usually light, but, go off in a few seconds, putting that problem in both the "electrical supply" and "gas supply" problem areas.
Sorry to be so vague, but, "long distance" troubleshooting can be a bear at times (no visual or auditory clues to work with).
Hope the above helps a little...