Yes, crank it up, watch the propane for the first tank to go empty as it's going to suck it down fast. Keep the heat going and get it thawed out then be sure it's covered until you know what is going on with the roof.
If your propane is low try to go to a propane place tomorrow so you can make it until Monday. Good propane places wont charge for a full fill if you have some propane left in the tank. They will only charge for what they put in. Don't buy rhino's at walmart. Pay attention to your change over valve, some auto switch to the opposite tank and you may not realize one tank is empty.
When changing tanks over, turn the valve on slowly or it may stay on red in the change over valve window. If it wants to stay on red, turn the valve off, turn on a stove burner and it will release the system pressure, then turn the burner off and again slowly open the propane tank valve. Tractor supply has good propane prices if you have one near that sells propane.