There are various pathogens that will cause dieback in Japanese maples but a very rapid decline, such as what you describe, is very often Verticillium wilt. For diagnosis, take a good sized sample of one of the dead portions (preferrably removed from close to the trunk) to your local extension office or better garden center that specializes in J. maples. If confirmed as VW, there is unfortunately nothing that can be done to save the tree and it is strongly recommended you do not replace it with another maple or VW-susceptible species. Vertcillium wilt with Japanese maples is pretty common in the NW :-(