The 16-year-old girl was left alone when everyone else went home after an under 16s Sixth Form disco.
Meanwhile, her anxious parents reported their daughter missing to police at 2.14am.
Officers were called out on Tuesday, when the girl failed to return home from Manhattan’s nightclub in Barrow, Cumbria.
But she didn’t wake up until staff had locked the doors and left and wasn’t discovered until 8.30am the next day, when cleaners arrived and let her out.
The girl attempted to get out of the venue but was unable to and was could not call for help because she had left her phone with a friend.
Friends and family had been searching for the girl all night after appeals on social media.
The teen’s mum, who didn ot want to be named, said: “We are relieved she is home safe and especially thankful to police who were very supportive and acted quickly.”
A spokesman from Manhattan’s said: “We’re unable to say too much at this stage but we had to make sure a young girl got home okay after finding her in the club after closing time.”
Cumbria Police have launched an investigation into the incident, a spokesman said their inquiries are ongoing.