Has the tree separated completely or is it an immediate danger to a structure? You might be able to save it by drilling completely through the trunk, inserting a heavy piece of all thread, and cranking it tight with some big washers and nuts. It completely depends on the individual situation, but that method can be successful if you catch it early. I wouldn't do it if it's near a house though. Insurance companies won't be amused if they come and find a doctored up tree laying on the house.