It was a Billy Joel state of mind at the opening night of the Tribeca Film Festival.

The world premiere of the Piano Man’s documentary “Billy Joel: And So It Goes” kicked off the 24th edition of the festival at the Beacon Theatre on Wednesday night.

Although the 76-year-old music legend was absent from his big night after revealing his shocking brain disorder diagnosis two weeks ago, Joel’s presence was still very much felt as the audience watched the first half of the two-part doc that will premiere on HBO and HBO Max later this summer.

Here, we break down some of the biggest revelations of Part 1:

While we always think of Joel as the ultimate piano balladeer, he actually wanted to get his Led Zeppelin on with an early band.

The young Piano Man broke away from his early band The Hassles with Jon Small to form the heavy-metal duo Attila (as in Attila the Hun) in 1969 that set out to “destroy the world with amplification.”

“I didn’t know one person that liked what we were doing,” says Joel, who can be seen as the hair apparent to Robert Plant in the doc. “But I did.”

The Piano Man fell in love with Elizabeth Weber, who was Small’s wife at the time.

“Billy one day came up to me … and said to me, ‘I’m in love with your wife,’ ” says Small. “So it was a very devastating thing for me, because these were basically my two best friends.”

“I felt very, very guilty about it. They had a child,” says Joel. “And I felt like a homewrecker.”

He continues, “I was just in love with a woman. And I got punched in the nose, which I deserved.”

But Small would eventually take the high road after Weber inspired Joel’s 1971 debut, “Cold Spring Harbor.”

“You could tell from the lyrics that he was really in love with Elizabeth,” Small says of songs such as “She’s Got a Way.”

Joel was “in a lot of pain” after confessing the affair.

“I had no place to live, I was sleeping in laundromats, and I was depressed,” he recalls, “to the point of almost being psychotic. So I think, ‘That’s it, I don’t wanna live anymore.’ I just didn’t wanna be. It was sorta like, ‘Why hang out? Tomorrow’s gonna be just like today is — and today sucks.’ “

“So I just thought I’d end it all,” he says of trying to take his own life by overdosing on sleeping pills. The singer would end up in a coma for days.

But after waking up in the hospital, “My first thought was, ‘You can’t even do this right. You can’t even do yourself in.’ So I thought, ‘I’m gonna try to do this again.’ ”

In his second suicide attempt, Joel drank a bottle of Lemon Fresh

“I was the one that had to get him in the hospital,” says Small.

“Even though our friendship was blowing up, Jon saved my life,” says Joel.

Feeling like “a lost soul,” he sought psychiatric help.

“So I checked into an observation ward,” says Joel, who then realized “these people have real deep psychiatric problems. I was just feeling sorry for myself. And I said, ‘That’s it, I’m not gonna ever feel that sorry for myself again.’ ”

After he moved cross-country to Los Angeles with Weber to make more music, Joel became the resident lounge lizard at the Executive Room in LA.

But he didn’t want to perform under his recording moniker, so the artist born William Martin Joel performed as “Bill Martin at the keyboards” after his first and middle names.

“This was a real-life experience,” he says. “And I thought, ‘I gotta get a song outta this!’ … And that’s where the ‘Piano Man’ song came from.”

When it was suggested that Joel use Elton John’s band, he thought that it was “a terrible idea.”

“Why would I wanna work with another well-known piano player’s band?” he says. “The lack of imagination was staggering. And I was already having critical issue with them comparing me to Elton.”

After Joel said goodbye to Hollywood and returned to New York with his now wife and soon-to-be manager Weber, he sought out famed Beatles producer George Martin to work on what would be 1977’s “The Stranger.”

Martin agreed, but he wanted Joel to use studio musicians instead of the New York crew he had formed on 1976’s “Turnstiles” — a dealbreaker for him.

“And I passed on working with George Martin,” he says. “Love me, love my band.”

“Just the Way You Are” — the second single from “The Stranger,” which would become a US Top 10 hit — was inspired by his wife and then manager Weber.

“It just didn’t feel right, it was too mushy,” says Joel.

But after being pushed to be a single by Weber — not to mention Linda Ronstadt and Phoebe Snow — his breakthrough ballad won Grammys for Record and Song of the Year and took Joel to a whole new level of stardom.

He was an arena headliner now.

The “Only the Good Die Young” lyric, “You Catholic girls start much too late,” drew the ire of the church.

“It ended up being banned by the Catholic church,” he says. “And then, as soon as they banned the sale of it, kids went out in droves [to buy it].”

As Joel’s drinking problem worsened, he took to riding his motorcycle while intoxicated. And when he drunkenly rode his bike to Weber one night, it inspired his 1980 hit “You May Be Right” with the lyrics “You may be right/I may be crazy/But it just may be a lunatic you’re looking for.”

But after Joel had an accident while riding his Harley-Davidson in 1978, it crashed their marriage.

“I was amazed that I was still alive,” he says. “I should’ve died in that accident.”