When a frame gets too narrow, then the three menus in the top left-hand corner of the frame stop drawing, to avoid them becoming too wide for the frame itself. To get them back, simply make the frames a little wider again.
As for the ‘Unlink’ button, if you have a music frame that’s inherited from the master page, i.e. its identifier begins with "M’, so it’s something like “MA”, then clicking the ‘Unlink’ button will disassociate it from the frame chain in the master page, and make it into a layout-specific frame, as if you had basically deleted the one from the master page and dragged out one onto the layout with exactly the same dimensions and position as the one from the master page. You can indeed reverse it, by choosing the original frame chain from the list in that menu again – e.g. if you “unlink” a frame from the “MA” frame chain it might become part of the “LH” frame chain, but you can then simply choose “MA” from the list again.