One guest was hospitalized in critical condition after falling two stories down the stairwell and the rest of the revelers found themselves stranded on the roof of the seven-story building.
"No one knew what to do when we saw the stairs had collapsed, we were just stuck," Hayley Salmon, 20, told the New York Daily News.
Firefighters arrived and deployed ladders and a cherry-picker to methodically get the stranded guests, most of whom were New York University students, off the roof.
City inspectors were on the scene Sunday trying to determine why the stairs collapsed. Salmon said the structure did not appear to be unsafe or in poor repair.
Officials told the New York Post that repairs to damaged landings were made and residents of the building were allowed to remain in their apartments.