I recently tried to “autohide” my desktop panel in KDE4 (Mepis 8.5, 64-bit). To my consternation, it would not “unhide” when I approached the bottom edge with my cursor, even after rebooting. I could see a little shadow, as though it was thinking about coming up, but it didn’t happen.
Here’s the fix, assuming you have some folder on your desktop that will allow you to open up Dolphin:
Navigate to /home//.kde/share/config/plasma-desktoprc
Look for a line in that file that says “panelVisibility=1”. Change it to “panelVisibility=0”
Save the file, and reboot (right-click on the desktop… it should give you an option to “Leave”)
That should get you going again… at least it did for me.