A gas water heater contains a pilot light, a little flame that stays on usually to mild the burner once the sign is shipped from the thermostat. Just before replacing a water heater thermostat, check it for continuity to make sure it's the perpetrator for the water heater not heating. https://leakfixsolutions.com/top-10-plumbing-repair-services-access-denied-for-top-notch-repairs/