I am a new Ubuntu user having a problem with my Ubuntu 14.04 panel icons not properly displaying. Before this happened I had a problem to boot a system - it took years to display anything after the login screen. The missing icons are for the network, locale, Thunderbird, power, sound, and system.
PS
If this is a duplicate of another question then sorry I couldn't find the right link, hope you to share a link.
sudo apt-get update && sudo apt-get upgrade
and see if it gives you any errors. – Ian May 04 '16 at 19:43unity --reset-icons
displayed icons in the login screen but it happened only there. And I have to reorder panel icons now ;) – Karol May 04 '16 at 20:02