The reverse gear inhibitor cable is probably snapped. Take the gaiter off the gearstick and look at the small cable running down the front of the stick. Give it a tug, if it's snapped then you need to spend around £40 at Fiat for a new one.
Hold the clutch pedal down for 2 seconds, with the car stationary, and then try. If it doesn't work, lift the clutch up and try again. Reverse doesn't like to be rushed