#1 Shut off the incoming water and the heater.
Should be able to just trip the lever back down.
Now it's time to figure out why it tripped.
Someone messed with the tempature and turned it all the way up?
If the temperature was set really high, turn that down and put up with the drip until the next day (allowing the temperature sensor to shrink). Then try pulling the lever to activate the pressure relief valve again and let the lever go quickly.
If that doesn't work then the valve is probably held ajar by a little sediment or rust and you will have to replace it.
The pressure relief valve should be tested every 6 months or so which usually prevents sediment from accumulating within it; some experts suggest replacing it every 7 years or so. When the water heater is replaced, the old relief valve should not be re-used.
You could get a trip under "normal" operation if you don't have an expansion tank, since water expands when heated and, because water is not compressible, something has to give.
In the home, the water heater temperature should never be set to "very hot" unless the heater is very old and even small amounts of usage give water that doesn't get very hot.
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