Clinton's Google account was hacked, and the password recovery was changed to another email address. Leo says that's why Google and Leo recommends 2 Factor Authentication so that he would be contacted should a password change happen. He can also use a secondary email. Clinton can contact Google and they can perhaps get his account back by answering questions that only he would know about.
He should keep in mind that if he used this as a recovery email for other sites, they are vulnerable as well. So he'll have to get it back ASAP before more damage is done.
Check out these articles on hacking for more guidance:
Kevin Roos: I dared two expert hackers to destroy my life. Here's what happened.
Mat Honan: How I Resurrected My Digital Life After an Epic Hacking
What we give away when we log on to a public Wi-Fi network