Can you hear anything coming from the roof area at all? Mine had both lift arms broken (severely), and you could hear the cable pulling the transport bridge back (and dragging half the arms) but the roof wouldn't move because it wasn't connected to anything but broken bits. The broken arms became more obvious when I tried to tilt it ... half would go up, crunching sounds rang out, and it wouldn't go back down without a fist pop on the roof cover. But if you're not hearing anything but the motor, and it isn't making any effort to move in any direction, I'd have to go with babymog's assessment on the drive cable ... although I'm not sure how much you can tell from feeling around with the panel in. You could try manually opening the roof from the trunk ... if the cable is intact and attached to the bridge, that function would work, right? If it doesn't then I think that points to the cable being the culprit.

I was kind of lucky that my problem was the arms and didn't have to deal with the cable, but disassembling and reassembling the roof was easier than I thought (time-consuming and patience-demanding, though). So you may not be up the creek.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)