The official cause of his death is listed as "debility" ( old age ) brought on by heart failure caused by arteriosclerosis. What is most interesting is that Adams died on July 4, 1826, the 50th anniversary of the Declaration of Independence and the same day that his friend, Thomas Jefferson, died. The Liberty Bell in Philadelphia was tolling, marking the deaths of both men, when it cracked.
I believe the portion of the previous answer about the Liberty Bell cracking when tolling the deaths of Adams and Jefferson is not correct.
The link below, which provides a detailed timeline of the bell's history, says the crack occured at least 15 years after Adams died.