I had the same thing happen to me and spent a long time trying to figure out how to release the spring loaded mechanism through a remote site (like the holes where the soft top supports sit when up.) I eventually placed the handle back into the original hole, wiggled it around some and it found its way back into the correct hole and as I turned the handle, the cover popped open! The nut that keeps the handle in the appropriate place had worked itself loose and was lying in the bottom of the tray. Needless to say, when replacing the nut, I used lock washers, and Lock-Tite to help prevent the mishap from recurring.
1969 280SL, manual, MB TEX, green/cognac