Per David at Power Probe:
Double check that the circuit breaker did not come loose.
Remove the black cover on the back of the Power Probe IV and check to see that the breaker has not slipped out of its socket.
If it gets knocked out, the probe will always display “Circuit Breaker Resetting”
It will also display that if the circuit breaker goes bad (breaker always open)
Replacement breaker are available.