You stop the X-server and run as root in order to repair an installation
--you can do this by running recovery mode from your grub menu or you could insert the live CD and boot the system
At which point I would use apt and reinstall the xcfe-panel..
--if you get that message about no xfce-panel is running and the desktop does not load
--if you haven't sone anything with the system yet, I would refresh it completely (a reinstall) to get a clean install
Your actual problem is not clear to me:
- Is it a desktop error on boot ?
- Have you tried to change an xfce setting or added a plugin to the panel (xfce-panel)
- Is it something else
--when you searched to find a fix, what were you trying to fix ?