Rain, a downdraft in a vent pipe, or a windy day can be enough to snuff out a water heaters pilot light. Relighting instructions are similar for most water heaters, and you can find them permanently affixed to the side of your unit.
In order to relight the pilot, remove the access cover to locate control knob. The knob should be in the “Pilot” position. While depressing the control knob, light the pilot light with a long match or wand lighter. Once the pilot ignites, continue holding the knob down for a full minute to bleed air out of the line. (For a water heater with an automatic igniter, the relighting process is virtually the same. Instead of using a lighter or match, though, you’ll push the striker knob repeatedly while depressing the control knob until the pilot ignites.)
Relighting the pilot light may or may not be all you need to do to start up your water heater once more. Depending on what happens after the pilot ignites, proceed with one of the following:
If the flame remains lit, you’re good to go!
If the pilot light flickers and goes out soon after relighting, clean the pilot orifice.
If you’re able to light the pilot light, but it goes out when you release the control knob, the thermocouple probably needs replacing.
Remember, if your water heater is not in use or won’t light, turn off the gas or propane.