If the breaker was on, and the undervoltage coil tripped it - it should now be open. If it is still in the closed position - you have a problem and I would suggest you check your tripping ciruit wiring.
Do you need to know how to re-close it? If so - you definately should not be working with electricity.
Ah. <...UV...> ≠ ultraviolet; <...UV...> = undervoltage. Got it now.
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