one of my chums had this exact problem and I advised him to try and fix it without taking the lever apart, on the basis that something had come adrift inside and it might be persuaded back in as easily. What I didn't expect to happen (or advise) was that tools were applied to the cable spool in order to force the mechanism backwards. The lever appeared to work again after this but rather ominously there was a fragment of metal that came out and the cable nipple had a gouge in it. After this the lever was tested and it didn't work properly.
I think what may have happened was that the cable had enough 'spring' that the nipple popped out of its seating in the spool whilst the cable was slack, and then protruded far enough that it stopped the spool from rotating. If the cable is allowed to go slack when the shifter is set in any other position than the one that is used to change the cable, there is presumably a danger that this might happen. [There is also a query in my mind about the exact nipple size and shape; some sizes/shapes may favour it popping out and not reseating itself.]
Anyway if this has happened you may be able to tease the nipple back into the spool, provided you can get access to it. I don't recommend doing what my chum did.
cheers