Unity Launcher is an application switcher and launcher in Ubuntu Unity.
In some cases, it goes in the background, behind all windows and the Unity Shell, and it's not possible to move it to the frontend, in front all other windows (what is its default and normal behaviour). This happens no matter whether you enabled Unity Launcher auto hide option or not.
I noticed that in the most cases GIMP causes this annoying Unity Launcher's behavior. After some further use, the computer could freeze for a brief moment and Unity Launcher and Dash could continue to work properly in front of windows and dash.
After rebooting, the issue dissapears until the next time (there is no rule for this bug)
How to bring Unity Launcher back to front without restarting the computer and destroying the session
Here are the steps how to restart unity launcher without destroying the session and avoiding restarting
1) Start terminal
2) Enter the following command:
3) Hit Enter button
Screen will flick a few times and after that everything will be back again.
Consider that the above steps is not the real bug's solution but rather a way to temporarily fix its outcome.