You have provided little to no info. Like What brand, what size, etc. With such limited info here are a couple of things to try. Be sure the burner and ignitor are free of spider webs. Be sure the heat tube thru the heater is clear. remove the burner and ignition parts and drag a small rag bundle thru the tube with a wire. Battery up. Turn on the gas and run the stove burners and all the other gas appliances too. After setting all winter the gas lines need to be filled with gas and the air pushed out.
On the electric if it is an Atwood there may be a 120 volt switch at the back of the heater. On a suburban the 120 v. switch is probably on the front of the heater. You'll see it when you open the outer door.