Firefighters were called to Ubox, on Church Street, Market Warsop, on Tuesday, June 14, at about 12.15pm.
A spokesman for Nottinghamshire Fire & Rescue Service said crews from Warsop, Edwinstowe and the aerial ladder platform attended and two firefighters in breathing apparatus located and extinguished a fire in the roof space.
Oli Cox, director of Ubox, which launched at the start of the year, said: “I am heartbroken and gutted. We opened as normal, and had a busy morning.
“We went to do cardio upstairs and found the place engulfed in smoke and flames and rang the fire brigade.
“It is the children's classroom that has gone up, where we help young offenders, people with habitual drink and drug problems, and also run mental health awareness courses.
“It is their room that has been destroyed. The fire is a result of a fault in the roof."
He said no-one was harmed in the fire and praised the emergency services.
The gym will have to close for a few days, but Ubox plans to run a Junior class at The Black Market Venue, on Warsop’s High Street.
Oli said: “Please rest assured that we will be open in the coming days.
“However the use of the cardio suite will be temporarily prohibited.”
When speaking about the damage, Oli said: "We have some water and smoke damage to deal with.
“And now we have to redo the hub and the room. We are going to set up a JustGiving page. We want to get people back in as soon as we can. If anyone wants to help out, they can come down and give us a hand. Just please keep supporting Ubox as we continue to support the community.
“It is a knock back for sure, but we are strong and we are here to stay.
“We fought and built this place from the ground up, starting with nothing. We are not going anywhere.”