Icons sometimes disappear from the Panel notification area when Ubuntu 14.04 starts up. Not the same icon disappears every time, and only one icon disappears at a time. This happens infrequently, and I have also seen it happen on older versions of Ubuntu. The next time I start up Ubuntu, all of the Panel icons are back in the notification area of the Panel where they should be.
Here are some examples of icons that have disappeared from the Panel notification area at startup.
- battery indicator
- network indicator
- Thunderbidownload managerrd Mail
- sound
- clock
- icon (gear in Unity / ⏻ in GNOME) for accessing functions such as Lock/Log Out/Suspend/Restart/Shut Down, etc. from a dropdown menu
All of these icons have disappeared from the Panel at least once, however the indicator icon for Psensor temperature monitor application which usually takes about one minute after startup to appear on the Panel notification area has never disappeared from the Panel at startup.
How can I make a missing Panel icon reappear without rebooting the operating system? I would prefer a general solution that works for any of the missing icons in the list.
, please follow these instructions. 
Best Answer
It happens to me to from time to time. I don't know the cause.
To make the missing panel indicators reappear without rebooting you can restart the indicators manually by running the following command in a terminal or with Alt+F2
The panel indicators will blink off for a second, and then all of the panel indicators will restart and immediately reappear.
In Ubuntu 15.04 and later restart the panel indicators manually by running the following command:
In Ubuntu 17.10 and later GNOME has replaced Unity as the default desktop environment. To make the missing panel indicators reappear without rebooting in Ubuntu 17.10 and later run the following command: