It could be one of there faults
1. The fan-control switch relay is blown
2. The fan control relay is blown (I think there are two relays, this being the second)
3. or the speed-control resistor has packed up.
1 & 2 seem most common on French cars
3 is more common on German cars.
Unfortunately it may mean dis-assembling the dash on the passenger side. I'd suggest if you are not a very competent DIY'er, you need a tech as there's at least one airbag on that side.