So was everyone else!
I found that deleting panel was a usual case. Inserting new panel did not solve my problem.
I found the following codes:
gconftool-2 --shutdownThis solved my problem of restoring the default panel, but I lost all my shortcuts that I had placed on them.
rm -rf ~/.gconf/apps/panel
I had to add to the panel, manually. Which is pain. But not more than losing them all!