Air Canada investigating after passenger left asleep on empty plane

A sleeping passenger was left on board an Air Canada flight earlier this month hours after the plane had landed and the crew disembarked.

Tiffani O'Brien, of Ontario, Canada, said she fell asleep in an empty row of seats on her short flight home from Quebec City to Toronto. She awoke hours later around midnight still strapped to her seat and all alone on a cold, dark plane.

"It was completely pitch black," O'Brien said in an interview Monday with CTV News. "I thought, 'This is a nightmare, this is not happening!'"

O'Brien said she texted her friend, Deanna Dale, who drove her to the airport in Quebec City earlier that day. Dale told CTV News she called customer service at Toronto Pearson International Airport to tell them her friend was trapped on the plane.

But then O'Brien's phone lost battery power and a "sheer sense of hopelessness" came over her, she told CTV News.

As panic began to set in, O'Brien said she entered the cockpit to search for something, anything that might help. She found a flashlight and turned it on, directing the light out of the windows of the plane in hopes someone would see it.

She then used the flashlight to find the main door of the plane and managed to get it open, but the drop to the tarmac below was too steep.

So she sat in the opening with her legs dangling out and flashed the light on the side of the plane to create a reflection, hoping it would catch someone's attention in the distance.

Eventually, a grounds crewman driving a luggage cart spotted her and helped her down.

O'Brien recounted the incident in a June 19 post shared by Dale on Air Canada's official Facebook page.

"I haven’t got much sleep since the reoccurring night terrors and waking up anxious and afraid I’m alone locked up someplace dark," O'Brien wrote.

An Air Canada spokesperson confirmed the incident to ABC News and said the airline is investigating.

"This customer was left on our aircraft after the flight," the spokesperson said in a statement Sunday. "We are still reviewing this matter so I have no additional details to share, but we have followed up with the customer and remain in contact with her."

ABC News' Brad Mielke and Barbara Rick contributed to this report.