My panel disappeared all of a sudden after I clicked on the calendar. Restarting KDE didn't bring it back. After some searching, I finally found the solution to this reportedly common bug. Part of the problem in finding a solution is that some people refer to the panel as "kicker". The bug can occur under different circumstances and apparently is not going to be fixed because KDE4 has a new panel. Since I had a hard time finding the solution, I'll just post it here to make it easier for the next person to find.
The bug causes a line in the /home/~/.kde/share/config/kickerrc file to be changed.
Open that file in an editor and look for the offending line which reads:
XineramaScreen=-1
All that is needed is to change that line to read:
XineramaScreen=0
Restart KDE and your panel will be back. I found that it was now in a different location. I have it autohiding in the top left, but it returned to the bottom left. No problem. Now that I can get to it, I just right click on it and choose "Configure panel".
PS: The bug is listed as Bug 127466: On auto-hide Kicker doesn't show anymore here:
https://bugs.kde.org/show_bug.cgi?id=127466
The bug causes a line in the /home/~/.kde/share/config/kickerrc file to be changed.
Open that file in an editor and look for the offending line which reads:
XineramaScreen=-1
All that is needed is to change that line to read:
XineramaScreen=0
Restart KDE and your panel will be back. I found that it was now in a different location. I have it autohiding in the top left, but it returned to the bottom left. No problem. Now that I can get to it, I just right click on it and choose "Configure panel".
PS: The bug is listed as Bug 127466: On auto-hide Kicker doesn't show anymore here:
https://bugs.kde.org/show_bug.cgi?id=127466