Electric water heater– turn off the electric breaker to the heater. You will burn up your heating elements.
The overflow is called the T&P relief valve. That has popped.
Push the flip lever back down and see if it blows again. If it does turn down the temp. If that does not fix it purchase a new one.
If the pipe coming out of the T&P is pvc-cut it in half, and also purchase a coupler of the same size(should be 3/4″). Unscrew the pipe from the old T&P. Unscrew the bad T&P–caution water may come out but only to that level. Screw the new t&p into the same place. Screw in the pipe you removed from the old t&p and then use the couple to attach the drain back together.
If PVC-use pvc glue
If copper-you will have to sweat the drain back together.