You could possibly have a broken cable or the fasteners could have fallen out. By closing it manually do you mean with your hands directly on the sunroof or do you mean by the emergency mechanism in the trunk. If I am not mistaken, the cable system is ok if it can be operated from the trunk. If this is the case, the the motor is probably at fault. If you can move the sunroof itself then the cable is disconnected or broken. The interior panel of the sunroof can be removed to verify this.
My 190 has the same sunroof mechansim as the 124 chassis as I have referenced a 124 shop manual to diagnose my problem. In my case the fasteners that connect the cable to the roof mechanism fell out. I replaced the fasteners with loctite and have not had a problem since. It was an easy repair.

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T