Question
I have three sets of on-off valves on a manual sprinkler system. Each set works a different zone in the front yard. I can't get the middle one to shut the sprinklers off.  I keep turning it off but nothing happens and the water stays on.  I had to shut the water off to the house.  How do I fix the one valve to get it to shut the water to its zone off.

Answer
I'm guessing you have either a ball valve (1/4 turn) or a gate valve, which is the type you turn a number of times to open or close. Either one can loose it's connection to the "ball or gate" and make it not function.
You will need to cut it out or thread it out and replace it. Or install a new valve just past the one that is not working, leaving the old on in place. Neither is an easy task.
I hope I guessed right about the style of valve, if not reply back with more info, and I'll try to help.

Matt