Electric heaters have a red RESET button and run off a breaker. Gas heaters have a pilot light and a gas control dial. The steps differ, so check which you have — if you're not sure, the label on the tank will say.
Make sure no one just used it all (back-to-back showers, laundry, dishwasher). A tank needs 20–40 minutes to reheat a full load. If it's only one faucet that's cold, the problem is that fixture, not the heater.
Find the water-heater breaker in your panel and push it fully OFF, then back ON. Give it 30–45 minutes to heat before judging.
On the tank, pop off the small upper access panel and fold back the insulation — there's a red RESET button. Press it firmly; you may hear a click. If it trips again, stop and put in a work order.
Make sure the gas control dial is set to ON (not PILOT or OFF). Look through the little window near the bottom for a small flame. If it's out and you know how to relight it per the label instructions, do so — but never if you smell gas.
If your gas stove or furnace is also out, it's a gas-supply issue for the whole home — contact the gas company.
You smell gas, the reset button keeps tripping, or you see water pooling around the base of the tank — leave gas alone and call us (911 or the gas company first for a gas smell).
Hot water's back — good. A tank that keeps tripping its reset, runs out fast, or leaks is near the end of its life; send a work order so we can get ahead of it.
Tell us if it's gas or electric and what you tried — and mention any water around the base, since that changes how quickly we come out.