Complex sentence: Another heart attack in November 2002 resulted in a surgery in 2003, and sealed any career plans he had, though he continued as guest singer on occasions, such as Bonnaroo Music Festival, 2004, his last public appearance.
Split into simpler sentences: Another heart attack in November 2002 resulted in a surgery in 2003, and sealed any career plans he had. Yet Burnside continued as guest singer on occasions, such as Bonnaroo Music Festival, 2004, his last public appearance.