Main water valve just turns, turns and turns

Having updated a lot of old homes this is not an unheard of problem. What I have found on old gate valves that in some cases they close and seal well but when opening the gate block stays in place and the threads are not quite engaging and lifting the gate block. I have been able to re engage the gate by pulling the valve stem and using a hook to pull the gate up (I have turned off well pumps and water mains to do this) put everything back together knowing the next time it was turned off it would do it again but that gave me time to find replacement parts.

So I believe the gate block dropped off the valve stem or the end was rotted off. In most of the cases I was able to find replacement parts. On several cases the design was different and the entire valve had to be replaced. But pulling the gate up got the water flowing. (After the valve was reassembled).