Put safety and water control ahead of finding the hidden cause.
The short answer
Stop using the fixture. If a known, accessible fixture or main shutoff can be operated safely, close it and confirm whether the flow changes. Stay away from wet electrical equipment, bulging ceilings, and unstable materials. A provider will need the water status before choosing an inspection.
What to notice
Containment without investigation
Use a container only where it can be placed without entering an unsafe area. Move dry belongings out of the path, but do not pull down wet ceilings or open walls. Photograph the visible water path from a safe place. Water can travel along framing and piping, so the stain is an observation, not proof of the source.
Describe what controls the water
Tell the provider which valve was used, how the valve behaved, and whether flow stopped fully. If a valve is stuck or its function is uncertain, do not force it. Note whether all fixtures lost supply or just one. That distinction can help identify the affected branch while avoiding a premature diagnosis.
Separate the repair from related damage
Plumbing repair can stop the source, while water removal, drying, electrical evaluation, and finish restoration may be separate scopes. Ask who is responsible for each. Contact the property owner or insurer when applicable, but do not delay immediate safety steps while waiting for a coverage decision.
One useful question
Should I keep turning the water on to show the leak?
Not if another test adds water or risk. Preserve the observation you already have and ask the provider whether controlled testing is needed. Tell them if the leak appears only during a particular appliance cycle or fixture use so they can prepare without repeated overflow.