Count your blessings, at least it isn't stuck OPEN.

I think the usual failure is the cable,  which has plastic beads on it
to allow it turn and slide... When the cable gets close to failure,  it
frays and parts of it protrude from the tube that carries it.  This
causes jamming.

It could also be that there is in fact something stopping the
retraction completely, like a screw that fell into the wrong space...

Do you hear a "clunk" when the roof stops opening? A strong magnet
might move something metallic out of the way if necessary.

You are going to have to open the roof to fix this, one way or the other...

If you can open it a crack,  you MIGHT be able to pull down the leading
edge of the inside sliding roof cover and take a look under there.

Using the hand crank,  or assisting the motor with some external hand
effort seem like the best bets.

Good Luck,
Marty

odb,
Not much help but I remember being in the same position many years ago.
The problem as I recall was cogs on the motor drive and the cable.
All I can say is that I replaced the offending parts without using a tin
opener.
As I said this was many years ago, I think I had to remove the motor unit
left hand side
facing the rear of the car, leaving the cable drive in situ.
Manipulation of the drive cable, I think gave me access to the sliding roof
unit.
regards
dave