Not too big a deal, probably the common problem of stripped gears. Depending on your brand (Fantastic Fan or Maxxfan), parts can be found and replaced without removing the fan.
First, I'd recommend removing the knob to determine it isn't spinning freely on the shaft. If it's not loose and is spinning the shaft when turned, then most likely it is the gearset that is bad.
If you don't have a parts manual for your fan, I would call tech support for the company who made it and ask them for the correct part number. You can ask to order it directly from them, or just search Amazon, Ebay, etc. for the best price.