Could be something else, but you might try sunroof lubricant.

MB sells a specific lubricant for the sunroof rails.

This happened to me once in my E320, took it to a local Mercedes independent, and after lubing the rails, it magically started working again.

__________________
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".