It sounds like cavity rot; remove a dead branch from the tree and sniff the cut end if it smells sweet then you might consider having the tree removed, if not than there may be a chance to save it. Make a mulch of old news papers and mild (whole is available) not too soggy, and wrap the trunk of the tree with this mulch and only replace when it becomes dry, the tree will heal itself, it can take up to 4 months or it can take less than 30 days to see the results.
-Birdy