I recently have experienced the same problem twice when upgrading two computers - once when upgrading Lubuntu 12.10 to 13.10 and again today when upgrading a second computer from 12.10 to 14.04.
In both cases after logging in the desktop opened to a blank wallpaper - no panel and none of my desktop folders etc. Right clicking the desktop brings up a menu with various options not all of which work. In both cases logging out and back into guest session shows the full desktop with panel and folders etc.
How can I fix this?