At least here in the U.S., the ceiling mounted ventilation fan (extractor in the U.K.?) is covered by a snap on plastic grid that is easily removed. Once it's removed, assure the accompanying light switch is in the off position and then using a vacuum cleaner with the soft brush tip attached to the hose, clean out all the accumulated debris. Rarely, but sometimes, this has clogged the small fan itself and may start to work again. Use caution for the exposed wiring and, if possible turn off the circuit breaker powering the device.
In most cases the little electric motor that drives the plastic vaned van has ceased working and needs to be replaced. Inside the same grille work I described should be a tag identifying the make and model which will assist in buying a replacement. You'll need an electrician, as already recommended if it comes to that. This is a lot less expensive that replacing the entire system...