Story highlights Tom Wagner, left on plane, calls girlfriend for help; she initially doesn't believe him

He says he slept through landing and woke up in a dark cabin

Wagner was traveling from Louisiana to California

United offers $250 voucher while ExpressJet says it will investigate what happened

Tom Wagner woke up Friday night and the plane cabin was dark. And empty. Completely empty.

He looked around from his window seat near the back of the jet and wondered, "What is going on?"

He tried the jet door. It was locked.

Not knowing what else to do, he phoned his girlfriend and said, "Debbie, you gotta call the airlines. I'm locked in the plane."

She didn't quite believe him and started laughing.

"I said I'm serious," Wagner explained Monday morning on CNN's "New Day," chuckling about the conversation. "She finally hung up and called the airlines."

About 30 minutes later, the door opened.

Wagner, from Louisiana, was on his way to California to see his sister. The plane had a layover in Houston, Wagner said, so he took his hat off and took a nap.

Apparently a very deep sleeper, he didn't wake up when the plane landed. And somehow when airline employees were doing their usual sweep of the plane, they missed him. Dozens of passengers left the plane and so did the flight crew.

Wagner said he walked around the plane in the dark trying to figure out the best thing to do.

He said eventually workers opened the plane doors and, startled, asked him what he was doing in there.

He tried to explain calmly that he'd been left behind.

After his ordeal, he's been talking about what happened.

"I talk to people, (and they say) like, man, I'd have freaked out!"

Wagner stayed in a hotel room, on the airline, before flying to California the next day. He said United gave him a $250 voucher.

The flight was operated by ExpressJet, a United partner.

"ExpressJet is investigating to determine how this occurred. We sincerely apologize for the inconvenience this caused for the passenger," the carrier said Saturday in a written statement.

ExpressJet told CNN affiliate KTRK there was a post-flight cabin inspection, but the company couldn't explain how Wagner was overlooked.