The glass might have come off the guide or the cable could have snapped.
The whole thing is supplied as one assembly (motor, cable and guide). I replaced both of my rear window units two weeks ago and fitted recon assemblies supplied by the local Alfa dealer. It cost me £65 + VAT per side for the parts (exchange price). Ebay etc flog them for nearer £30, but I would definitely go for recon units.
Basically you remove the door card (search this forum for an excellent guide), pop the retaining plastic clip that holds the glass to the guide, remove two nuts that hold the guide and three that hold the motor and the whole lot then comes out (motor, cable and guide).
The first side took me 90 mins, the second side took 30 mins! It's easier if you have a new assembly in front of you to look at, otherwise everything is hidden in the door and it's hard to see exactly what it looks like.