I have a Hydro flame Atwood 8531 that stopped working during the night, was working fine before. Propane had?

I have a Hydro flame Atwood 8531 that stopped working during the night, was working fine before. Propane had about a 1/4 of a take left husband changed it over and when we tried to start it, it wouldn't and all we could here was a shhhhhhhh noise and nothing else. We did turn off the furnace and wait but still nothing Does anyone know what it could be... HELP PLEASE..