Take a breath. A flooded garage feels catastrophic and usually is not — most of the damage happens in the first twenty minutes, and you control most of those minutes. Here is the plan, in order. Do not skip ahead.
Minutes 0–2: stop the water
Find the cold-water shutoff on top of the heater and turn it clockwise until it stops. If that valve is stuck — old gate valves seize — go to the main house shutoff instead. Stopping the inflow is the entire game; a 50-gallon tank cannot flood your garage nearly as fast as an open supply line can.
Minutes 2–5: kill the heat, save your stuff
Gas: turn the control knob to pilot or off. Electric: flip the water heater’s dedicated breaker. Then move boxes, tools, electronics — anything you care about — out of the water’s path and onto higher ground. Do not wade through standing water to reach a breaker box; if water is near electrical, stay clear and call.
What NOT to do
Do not try to drain 50 gallons through a garden hose while panicking — you will be there for hours and the drain valve may fail mid-job. Do not touch electrical components while standing in water. Do not relight a pilot or flip the breaker back on ‘to see if it still works.’ And do not start disassembling things; note where the water is coming from instead.
After the water stops
Squeegee or push water toward the garage door, get towels on what remains, and get air moving with fans. Check where drywall meets the slab — that base stays wet longest and grows things you do not want. Take a photo of where the leak appears to originate; it helps the diagnosis enormously.
What probably caused it
The usual suspects for a sudden flood: a burst flex connector or fitting, a TPR valve stuck open, or a tank that finally let go at the bottom seam. A technician confirms which in minutes — the fix ranges from a fitting replacement to a new heater, and you will get the straight answer either way. Call (833) 713-2101 once the water is stopped.