This may be relevant to others but to be exact, I am using excel for Mac 2011.

The top Ribbon kept vanishing for no obvious reason, and the spreadsheet would be welded to the Mac Toolbar. Then I discovered one day that if I use 'Save As' for a spreadsheet and then select the larger 'Save As' window (by toggling using the button to the right of the 'Save As' field), my top ribbon would vanish.

To make it return, select 'Save As' and toggle back to the small 'Save As' window.....as the 'Save As' window reduces, the top ribbon returns and the excel sheet can be moved around the screen again.