The first thing to check on the stop valve is the packing nut. Just under the handle is a small nut that tightens the packing against the stem. When turning on the handle often times, the packing will simply begin leaking. A small turn with a wrench will usually stop the leaking if it is coming from the stem attached to the handle which passes through the body of the faucet. The only thing that keeps the water inside the faucet is the packing, which is usually a graphite or compressive material. If this doesn't work, then call a technician.