Stop the water and heater only with safe access
Once you can act without entering the affected area or approaching electrical danger, close the water heater's known cold-water supply valve if you can identify and reach it safely. If you cannot, have a qualified provider stop the supply. Tell them that the water is reaching ceiling lights.
The leaking heater also needs its energy supply safely isolated. An electric unit needs the appropriate electrical isolation; a gas unit needs its model-appropriate shutdown by someone who knows the equipment. Do not enter a wet area, open electrical panels, manipulate unfamiliar gas controls, or drain hot water yourself. A.O. Smith advises stopping the energy and water supply to a leaking heater and arranging authorized service. A.O. Smith leaking water-heater guidance.
Arrange the right help
We would make three responsibilities clear: a qualified electrician addresses the affected electrical system, a qualified water-heater provider finds and repairs the leak, and an appropriate water-damage provider assesses wet ceiling and building materials. Stopping the leak does not establish that the light or ceiling is safe.
Do not stand under a sagging ceiling or puncture it to release water. Take photos only from a safe location. Tell the providers when it started, what you shut off, and where water appeared. Keep the heater off until the cause is corrected and the provider confirms it can return to service. Electrical reenergizing needs the electrician's clearance separately.
Your next step: Keep the area clear, obtain safe isolation and urgent leak service, and coordinate electrical and water-damage assessment before using the affected space again.