If you are experiencing a leak in your unit you must contact your own plumber to respond.  Your plumber will be able to quickly and effectively determine where the leak is coming from and how to repair the problem.  If the leak is from a neighboring unit you should attempt to reach your neighbor and have them shut off the water to their unit to minimize damage.  Most shut off valves are located in the utility closet behind the hot water heater; however, if there have been any plumbing changes in the unit by a current or previous Co-Owner the valve may have been moved.  If the neighbor is not reachable or unable to shut off the water you should contact the fire department immediately and they will report and turn the water off at no cost.  You are encouraged to contact your neighbor directly or if that is not possible, file a claim through your homeowner’s insurance company.