furnace

T-stat....
Select HEAT
When you move temp set point UP (higher then room temp to HEAT room up) do you hear a 'click' at the t-stat?

If yes......
Then furnace fan should come on and blow cold air for 30-90 seconds (purge cycle)then propane ignitor should fire (Click, Click, Click at furnace) and burner light off

No click at t-stat...
Feeder DC Fuse bad. It would be in the DC Dist Panel...individual fuse 7.5A minimum or a 15A fuse (test it don't just look at it)

Click at t-stat but no furnace fan
Then the On/Off Circuit Breaker on furnace frame may need reset (open furnace access panel and push button in to reset)

OR time delay relay is bad (either a stand alone component or incorporated on to circuit board ---blower power)

DC comes from feeder fuse to timed delay relay/blower power on circuit board and to t-stat.
DC power to fan goes thru relay/blower power WHEN triggered by DC from t-stat

For specifics post Brand/Model of furnace

Last one I fixed did what OB said fan came on no fire installed new circuit board and all was well
agree with OB on resettable 12V circuit breaker I never thought about it have a reset breaker on the furnace itself ?? maybe good to take a check on that

Rest fuse on furnace. It's a push button type fuse