In fact, the Rock and Roll Hall of Famer said he didn't even know what to do next.
"I was depressed. You would be. You were breaking from your lifelong friends," he told BBC Radio 4's Mastertapes.
McCartney also addressed the reason for the split, saying, "The business thing split us apart," referring to a new manager and new business practices for the release of "Let It Be," the band's final record together.
After the breakup, McCartney added, he wasn't sure "whether I was still going to continue in music." Fortunately for the music world, he ended up recording upward of 25 studio albums post-Beatles.
He dropped his most recent album, "New," in 2013, and he has worked with modern greats like Kanye West, about whom McCartney said, "People says he's eccentric ... which you'd have to agree with. He's a monster. He's a crazy guy that comes up with great stuff."
McCartney said that over the years, he kept in touch with fellow Beatle John Lennon before his death in 1980.
"I would make calls to John occasionally. We just talked kids and baking bread," he added.