when turned on does the fan come on?
Do you hear a TIC, TIC,TIC as the igniter tries to light
Is there a battery
in your thermostat.
when was the last time you have turned on the heater and it has ran properly?
is the gas tank full and working properly. test the stove with all burners lit to check propane
flow and pressure. this will also help purge the line.
is the gas going to the heater turned on.
is their a separate power on switch at the heater under the cover.
are you plugged into 120 volt ac or running on battery
if running on battery is your battery voltage over 12.5 volts?
If voltage at the heater is too low the fan wont turn fast enough to make contact at the sail switch to send power to the gas solenoid.
Model and brand of your heater?