The Presidential Succession Act of 1947 states what would happen if the President and VP are dead, ill or can’t perform their duties.
The line of succession is the Speaker of House, President Pro Tempore, Secretary of State, State, Security of Treasury, Security of Defense, Attorney General, Secretary Labor, Secretary of Agriculture, Secretary of Commerce.
The 25th amendment allows for the Vice President to become president in the event of death, resignation, removal from office or impairment that prevents the current president from fulfilling his or her duties. They can get their job back once they are healthy again. An example is when Ronald Reagan was under anesthesia George W. Bush took over his duties.
Mike Pence, current Vice President of the United States
Formal duties of the VP: President of Senate and votes in case of tie, becomes President if President dies and can’t perform duties, VP presides over joint session of Congress to count votes for electoral college
Informal duties of the VP: attends state events, works with Congress to pass legislation, advises President