If the shaft isn't turning (but the handle is), remove the screw in the top of the handle and remove the handle. Either the handle, the top of the shaft, or both will likely show evidence of being rounded off.
(The valve, or at least the handle will have to be replaced if this is the case. It may be cheaper to replace the tank and all).
All is not lost, however, the gas can be used.
With the tank connected to the regulator hose (cause you don't really want to dump all that gas to atmosphere--that could be a problem), use a pair of vice-grips clamped on the shaft of the valve to turn the shaft and open the valve. (lefty loosey) Use the vice grips to close the valve (righty tighty) when you are done.
Lather, rinse, repeat until you have used the contents of the tank.
“cause you don’t really want to dump all that gas to atmosphere—that could be a problem”
Gunner’d do it...and have a party to celebrate doing it...:)