CNBC.com has a slideshow which mentions ten musicians who in most cases were at one time huge stars but at the time of their death had faded and were again very popular.
Among the musicians include Michael Jackson whose record sales and general interest was huge following his death when his prior album had not been successful and he both led the dead celebrity list as well as made more money than he ever had while alive.
John Lennon obviously he was one of the biggest stars ever with the Beatles but following the breakup of the group was not as popular as he had been until he was shot and his star again rose as everyone bought his records and remembered him. He continues to be one of the biggest stars ever. Johnny Cash, Roy Orbison, Ray Charles were also on the list as having faded then huge again after their passing and Kurt Cobain and Nirvana were still popular when he killed himself but became even more successful afterwards.