As regular readers may know, I have recently installed Xubuntu onto my Acer Aspire One.
On booting up a few days ago, my top panel had completely disappeared. Oops. Luckily, the internet in it's infinite dumb wisdom had the answer.
Alt+F2, run a Terminal, and then enter the command xfce4-panel &.
Many thanks and for full details, see here on the Ubuntu forums. All you guys rock big time :).