Check the 25 amp fuse FIRST.

Remove the rear left side panel to expose the sunroof drive motor assembly (there may be a small access panel for this purpose). See below.

Yours is a wagon right? If not, look in the trunk on the upper left side, at the top, near the front (you may have to pull the top of the side panel a little to expose the drive and you will seea a large plastic nut 21mm(?) on the drive assembly, use a socket turn the nut to close/open the roof in emergency.

See link for further help. There is a clutch that can be adjusted (only uf that is the problem)

http://mb.braingears.com/123_DISK2/program/Chassis/77-190.pdf

Hope this helps,
Acky