Blury beat me, the problem surely was you non volunteering saved your session, since xfce do that by default. So the problem sticks on future login and even in reboots.
Be sure to check the "save session" box when logout, so it will be there in the next login. Once the panel is back, I recommend to disable the "save session" thing. It causes more problems than solutions. You can do that from 'Settings' > 'Sessions and startup settings'. Uncheck the logout settings boxes.