A 73-year-old Bronx woman died Friday after her portable oxygen breathing tank gave out while she was trudging up 18 flights of stairs during a power outage in her building, officials said.

The woman, whose identity was not revealed, was just one floor away from reaching her apartment on De Kruif Place in the sprawling Co-Op City complex when her oxygen tank had emptied and she fell down the stairs and collapsed around 2 p.m., according to the NY Post. The elevator in the woman's 33-story building was not working after the power outage that was first reported at around 10 a.m. on Friday, leaving thousands of residents without heat, light, and water in buildings 1 through 8. The outage also knocked out service to the nearby Dreiser Loop Shopping Center, according to Co-Op City police.

The power outage was caused by a transformer fire in the building’s basement, with FDNY crews arriving to the scene just after 10 a.m, and knocking out the flames in two hours.

Police believe the power outage was the reason the the elderly woman took the stairs to her apartment.

As utility crews and repair workers fanned across the building complex to restore power, building managers opened up community rooms, converting them into heating centers so residents could stay warm while also powering up their devices.

Co-Op City is called the largest naturally occurring retirement community in America, largely populated by Bronxites age 65 and over.

"Some of them are sick, so I'm hoping they're being cared for properly," Asia Gray, a tenant in one of the impacted buildings, told WABC-TV.

According to a tweet posted by Co-Op City police, power was fully restored on Saturday at 6 a.m.