You can try a gearbox reset, It's in the owners manual.
Hints: before disconnection the battery, close all the electric windows and doors remove the key but don't lock the car. Wait 60 seconds before disconnecting the battery. Disconnect the positive of the battery for a minute or so (not important) re-connect the battery. Again, wait 60 seconds before opening the doors. Make sure there is a "-" in the gear display before continuing. If the gear display shows a letter or number, do it all again!
Realistically sensodrive gearboxes are a nightmare. My wife had a 2004 1.4 16V sensodrive. You may find a gearbox reset as stated above helps but problems will only get worse. To me it seems that the gearbox actuator puts extra wear on the gear selector. I had to rebuild the gearbox on my wife's due to it losing all gears. At the time I found the input shaft bearing had gone too & was causing a whining noise slowing down. The bearing kit cost £100 & took me a few evenings to fit especially difficult was the cleaning of all the parts as the bearing cage was clogging up oilways. However, the main problem was one of the gear selectors breaking a rivet meaning the selector passed behind & locked the selector up. This was re-rivetted. While you are there you really need to change the clucth as the sensodrives eat them. Fianlly you put it all back together & ind the clutch acuator fails. It's a waste of time fitting a 2nd hand one as it too will be knackered. Even doing it DIY you are looking at £350 of parts.
Hopefully the gearbox reset works for you but my advice to you either way is get rid of the car!
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