look in your trunk closely, the little leaf-spring that actuates the switch has come off. Its about 1 inch long and looks like the silver clip used on an ink pen to clip to your shirt pocket. when you lower the shelf it presses this leaf-spring against the button and the top cannot be lowered, when you raise the shelf the leaf-spring is released and the top can operated. Seems that if you keep the shelf in the lowered position its more prone (because of bumps/potholes) to break the leaf-spring 
Click on the link below its number 7
http://www.realoem.com/bmw/showparts.do ... g=61&fg=35
Click on the link below its number 7
http://www.realoem.com/bmw/showparts.do ... g=61&fg=35