What happens if the president is in a coma?

Answer 1

By 25th amendment, If a president is unable to discharge his duties, then he or his vice president has to inform in writing that president is unable to discharge his functions to vice president, speaker of house of representatives and president pro tempore of senate, vice president will become acting president till such time president is able to discharge his duties. Then president will give in writing to vice president, speaker of house of representatives, and president pro tempore of senate that hi is fit to discharge his duties, and will resume being president

Answer 2

According to Section 4 of the 25th Amendment to the U.S. Constitution, the Vice President and a majority of the President's Cabinet Secretaries inform the President Pro Tempore of the U.S. Senate and the Speaker of the U.S. House of Representatives in writing that the President is "unable to discharge the powers and duties of his office." The Vice President immediately becomes Acting President and remains Acting President until the President informs the President Pro Tempore of the U.S. Senate and the Speaker of the U.S. House of Representatives in writing that he/she has no disability. If the Vice President and a majority of the President's Cabinet Secretaries again inform the heads of the House and Senate in writing that the President is disabled within four days of the President's written declaration, Congress begins the process of deciding the matter.