HOUSTON -

It may be the worst flight ever -- a man traveling on board a United Express jet from Louisiana to California has a layover in Houston that ended up being much longer, darker and colder than he expected.

Tom Wagner says when he awoke from a nap he discovered he was cold and all alone and locked inside the plane.

"I looked up at the ceiling and saw the lights were out and looked down the aisle and nobody was home," Wagner said.

Somehow he had slept as other passengers disembarked the plane and no one noticed. Fortunately, Wagner says his cell phone was working and he was able to call his girlfriend, who then contacted the airlines.

"She started laughing. 'You better call United and get me off this plane!'" said Wagner.

Here's what Local 2 has learned: There was only one flight attendant on board Flight 4245. Express-Jet tells us it was that flight attendant's responsibility to sweep the plane or to make sure everyone was off before locking up for the night.

A spokesperson for the airline said, "ExpressJet is investigating to determine how this occurred." However, another source close to the airline says the flight attendant claims she walked up and down the plane and no one was on board.

Both sides agree the cleaning crew who unlocked the door about 40 minutes after wagner woke up... Were just as surprised as he was to find him on board.

Wagner said, "They said, 'Where's your badge?' I said, 'Dude, I don't work here. I'm a passenger on this airplane.' And he said, 'Hold on, hold on.' And he didn't believe it."

Express-Jet and United apologized to Wagner; they gave him a $250 voucher and a hotel room in Houston for the night.

A spokesperson with the FAA told us that the agency is following up with the airline, but would not call it an investigation at this point.

Wagner tells NBC News on the way back to Louisiana, he'll connect in Houston again. But this time he says, he will be well-rested.