I just restored the left panel of Ubuntu using the post: Unity doesn't load, no Launcher, no Dash appears, but the icons on the top-right corner in the Panel are still invisible.
I have searched but I haven't found anything that works for me on Ubuntu 14.04.
By an accident I am unaware of, I selected lxdm as the display manager so an ugly blue login page comes out every time I tried to log in, as described in this question: How to remove the ugly blue log in?. Now I run sudo dpkg-reconfigure lightdm and select lightdm as the display manager, all things I am familiar with come back again, including the lovely icons in the topbar.
PS: by executing sudo apt-get remove lxdm, we can completely remove that ugly blue login window forever.
Thank Seth for his help.
