Two ambulances and at least three police vehicles were scrambled to the restaurant on High Street at 2pm on Sunday, a witness said.
Police confirmed a woman was taken to hospital, and it is understood she had gone into cardiac arrest.
A spokeswoman for South Yorkshire Police said: “Officers provided assistance with crowd management while paramedics from Yorkshire Ambulance Service carried out their work.
“A 23-year-old woman was taken to hospital by ambulance.”
A diner, who asked not to be named, told how she was there with her children when customers were told to leave.
“We were going to get a meal and there was someone laid on the floor – I couldn’t see if it was a man or a woman,” she said.
“It was very busy and the manager was running everywhere and said ‘can you all evacuate’.
“When we walked past we saw paramedics trying to resuscitate the person who was on the floor at the top of the first set of stairs coming up from the ground floor.
“As I came out, I saw two ladies crying, one of whom was sat with a police officer inside.
“There were at least two ambulances, two police cars, and a police motorbike.”
The store was open again as normal, with no sign of police or paramedics, by 4pm.