If the president is unable to be president our vice president takes over. Than Speaker of the House, etc. We have a long list. Like if something happened to Bush and Cheney right now, a woman will become president of the United States, Nancy Pelosi. When sworn in you are just "assuming" presidential duties until the president is back. If the Vice President was out of reach, they can wait until the next day, but until the Vice President is found our House of Representatives' Speaker of the House will be "acting president".
http://en.wikipedia.org/wiki/United_States_presidential_line_of_succession