I agree on the detent spring, but probably stretch rather than broken. If it is broken then you will probably end up stuck in one gear. Which gear that you are stuck in will depend on your luck. Mine was stuck in 1st, but it happened about two miles from home so it wasn't too bad. Either way, the fix is the same - replace the spring. It is about three hours work if you have the tools.
Check out some of the clutch threads before you go in. Warp9.9 has posted a good mod for the clutch return spring which makes life easier and is probably better than the OEM setup. You may also want to check your clutch plates and steels while you are in there. If the clutches need replacing then I would strongly suggest looking at MRC (I think the name is right - no doubt warp will correct me if I am wrong) plates.