most likely to be one of the connector rods come loose, or the plastic connector that joins it all up has broken, easily done if the door has frozen up.
To look at it you need to remove the door card, which is a it if a fiddle. Pop off the plugs in the door handle and undo the bolts there, pop of the square-ish plug by the interior door handle to reveal another screw there, then most difficult of all, remove the speaker grille, the plastic tabs clip on, but getting them off in one piece needs care. There may be IIRC a couple of screws at the bottom of the door card, undo them, then pop the door card off the door with a spreader or large screwdriver, something that spreads the load, small and sharp will cause damage. Once the poppers are all undone lift the card upwards off the top channel.
You can now access the lock by peeling back the plastic cover and begin to see what the problem is.
IIRC there is a how to on the 145.com site, but you will need to register to see it