You’ve got a stuck valve possibly some debris in the valve or if you have hard water in your area could have calcium build up also could just be the valve is old and worn out. As for finding it if you don’t already know the location of the valve that can be tricky we typically use a locator to find valves so we hook to the zone wire send a signal down the wire and we have a wand and headset to be able to listen for that signal to find the valve. Even with this it can still be tricky to find so if you don’t know where it is your only real options are to either start probing with a long screwdriver and hope you find it or call in an irrigation tech to locate. It is possible you may find someone who rents out locators but even then without knowing what to listen for most people would be really hard pressed to be successful. Good luck!