The answer to 'shut-off valve not working' is that the shut-off valve may not be working properly due to a variety of reasons, including mineral buildup, corrosion, or physical damage. Symptoms may include water leaking around the valve, the valve being stuck in the open or closed position, or the valve simply not shutting off the water flow completely.
In many cases, the issue can be resolved by attempting to manually operate the valve or by cleaning and maintaining it. However, if the valve is severely damaged or cannot be repaired, it may need to be replaced by a professional plumber. It's generally best to call a licensed plumber if you're not comfortable working on the shut-off valve yourself, as improper repairs can lead to further damage or water leaks.