Most water heaters don't quit without notice. The hot water runs out sooner than it used to. The tank starts making noise. Rust shows up at the base, or the water comes out tinted. In homes from Carlton Hills to Riverwalk, those signs often show up before the tank stops working, and that's the best time to call.
Catching it early gives you options. A bad thermostat, element, gas valve or relief valve on a tank that's otherwise sound is usually a repair. A tank that's rusting through or leaking from the body usually means a replacement. If it's replacement time, the new heater gets sized to how many people live in the house now, and the contractor can talk through venting and earthquake straps.
You don't need to know what's wrong before you call. Tell us what the heater is doing, and the contractor can figure out the rest and explain it in plain English. If it's a small fix, you'll hear that. If it needs replacing, you'll hear the options, tank or tankless, and the decision is yours.