Five fire engines attended the blaze at Pizza Place, on Newcastle Street, in the early hours of Wednesday.
They were called at 4.50am when a resident on nearby King Street dialled 999.
Crews from Worksop, Retford and Harworth tackled the fire in the ground floor property.
It caused severe damage but no-one was injured.
“We are currently damping down and waiting for scenes of crime and fire investigation officers to arrive,” said watch manager Ian Cooper at 8.45am on Wednesday.
“We don’t yet know if it was deliberate or accidental.”
Newcastle Street was closed for a time during the early hours but reopened before rush hour. King Street remained closed to traffic for much of the day while investigations continued.
Parvis Ahmad has been running Pizza Place for the last three months. He was visibly upset as he surveyed the scene, and was consoled by his friends.
“We closed the shop at midnight and I got a call from a friend at 7am to say the shop was on fire,” he said.
“I got here and all I could see was fire. Everything was on fire.”
“Nobody lives in the property so no-one was hurt but I have lost everything.”
A friend added: “He has got to wait now to see if the investigation can tell us what happened.”
“They have spent a lot of money on it. It’s devastating.”
Neil Goodbold, who runs Attic Antiques on the same row, said there had been no damage done to his shop.
“They have told me I can’t open up today as they need to investigate and make everything safe,” he said.
“I feel so sorry for them. They have lost everything.”
“They are the first people who seem to have really tried to make a go of that place in a while. They are really nice people.”
Watch manager Cooper said the investigation into the cause of the fire could be a ‘lengthy’ one.
“We had five engines on the scene to start with because of the risk of the fire spreading to other properties,” he said. “It has caused severe damage to the back end of the property and there is smoke damage to the rest of the shop,” he said.
As an additional fire safety reminder, he also urged people to be careful with Christmas lights at this time of year. “Don’t overload sockets - one plug, one socket. And check your smoke alarm is working,” he added.