Since the Constitution does not indicate an order of succession beyond what is written in the 25th Amendment, Congress passed the Presidential Succession Act in 1792. It was amended in 1947 giving us the following order of succession:TWENTY-FIFTH AMENDMENT
SECTION 1. In case of the removal of the President from office or of his death or resignation, the Vice President shall become President.
SECTION 2. Whenever there is a vacancy in the office of the Vice President, the President shall nominate a Vice President who shall take office upon confirmation by a majority vote of both Houses of Congress .