Where did Dr.Crusher have to go ?
On a years leave of absence.
Gates McFadden did take a year off (I think it was to have a baby) but the reason they gave for the character's abscence was that Dr. Crusher had been offered the post of head of Starfleet Medical, and she decided to accept it. Dr. Pulaski came in her place, but after a year away, Dr. Crusher decided she wanted to come back. ( I don't think they were really specific about why she wanted to come back; maybe just to be with Wesley). So, Dr. Pulaski left. For more information, pick up a copy of Star Trek: The Next Generation Companion it has a lot of information in it.
During the second season of STNG, Dr. Pulaski comes on board the Enterprise as new Chief Medical Officer while Dr. Crusher leaves to become the head of Starfleet Medical. This is mentioned during the episode, The Child.
