What specific work were they doing when power was lost?
It could be they had their power equipment plugged into this circuit and it just tripped a breaker. (Things like air compressors can use a lot of power!)
Also contractors extention cords will become worn and tend to trip GFI breakers.
Or if they were drilling / sawing into the wall, they could have cut a wire.
If nailing, they might have punctured a wire in the wall with a nail.
Or there could be a loose connection somewhere and they could have been just hammering and this caused the connection to break.