Aside from the pressure safety switch in the propane tank, there is also a check-valve in the end of the hose connection. Turn of the tanks and allow them to reset. Also, disconnect the hose, shake the end of the hose to loosen the check valve and re-connect. S-l-o-w-l-y turn the propane back on. This will hopefully get the gas flowing.
Another thing: If it is real cold out, the propane can freeze at the connection. High pressure going to low pressure makes things colder. Pure a large cup of hot water over the connection. Where to get hot water if you can't get gas to heat it? It is a chicken and the egg scenario for sure.