Be careful when you open the BC; it's not the easiest thing!
Attached is a photo of the insides. (The front has a hinge and the top opens like this when some screws are removed; instructions are on the bottom of the BC).
On the top-left corner, where the blue wires go, is the cd-covers driver motor. There is a small (almost visible, from the bottom of the motor to the right) belt that turns the big vertical axel (and there go some white strings that slide the actual cover). By checking the belt and trying to turn the axel by hand you should be able to pinpoint where the trouble is. Of course there is the cassette cover mech where you can compare how it should work (or "borrow" the belt for the cd cover... :)
Bang & Olufsen - The art of controlling sound, picture and light