Is it fed from a seperate feed from the house via breaker, can you check this too?
You may find that if the supply is not from your house then there may be a breaker off somewhere else.
Do you have a meter to test the incoming supply?
It could be a council supply for a rented garage?
It may be as part of the rates and not metered
Do you have a multimeter to see if it is still a live supply?
One that you would test voltage with - your own test meter?