Is the handle tight on the valve? Is it possible the nut holding the handle to the valve stem has backed off to the point the handle is no longer engaging the valve stem to allow it to rotate the ball inside the valve?
If the handle is tight sounds like you may need to replace the valve.
I haven't looked at mine to know whether its a gate valve, ball valve, etc... shouldn't make any difference as the handles of all valves are connected to stems which move the internals of the valve.
US Navy Vet, Liberty Tree Member of Oath Keepers, NRA & VFW Life Member, Alaska EMT.
2009 Safari Cheetah 40 SKQ
2009 Winnebago Chalet 231CR