the window regulator is the mechanical structure that goes between the motor and the window itself.usually a conglomeration of cables gears and springs. pleanty of these components can fail without notice. i would take your door apart and see if its something simple. roll it up and down a few times so you can see how that particular one works. most commonly on benzes ive seen these nylon siding
fittings on the base of the window crack in half which causes the problem you described. whole window regulators are inexpensive and easy to replace. i paid about $120 dollars for one a few months ago for my 300E. it included a motor also. hope this helps!
good luck,
adam

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Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!)

PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320