There are two buttons on GFCI devices. You can test the device by pressing the black button. That should cause the circuit to trip, indicated by the red button popping out. When this happens, the outlet will be disabled and all other outlets and electrical connections on that circuit will be disabled. You reset the device by pressing the red button until it latches in place. If the device tripped by itself, check for causes that might have had it trip before trying to reset it.