I installed gnome-session-flashback for a test. It messed up the top bar of Mate Desktop. Currently, the button in the top-right with the system monitor/shutdown is missing, the clock is missing, there is no battery indicator, or sound.
The "mate-panel --reset" command does not help.
Is there a command to reinstall the desktop and reset it to its original state?
Not sure if you have mate-tweak installed, if so you could go control panel, mate-tweak and select another panel (traditional, familiar, etc). That should rebuild the panel.
Thank you for taking the time to respond to me. I tried this trick, but it did not work for me.
I have uploaded a picture of the top bar as I see it.
Hi @richterbg ( Iliya petkov) 
You wrote:
Since you've mentioned that @pavlos_kairis helpful suggestion of using MATE Tweak ("mate-tweak") and changing the Panel Layout to another panel (layout) did not solve your problem of some indicators missing from the top horizontal bar (clock, battery indicator, sound ...), then I suggest that you try the following:
1 - Open the "Indicators" program (click on the Ubuntu MATE Menu, hover your mouse over the "Preferences" menu to show its options, and then click on the "Indicators" icon in the right column), as I show in the following screenshot:
2 - That should have opened the "Ayatana Indicator Settings" window. For instance, for the clock, click on the "Date and Time" section and check that the options that you want to display (for year, month and date, day of the week, etc ...) are enabled and that the "Enable this indicator on login" button at the bottom of that window is also enabled (dark), as I show in the following screenshot:
3 - Do the same thing for the "Sound" section/category, that is, verify that the "Enable this indicator on login" button at the bottom of that window is also enabled (dark), as I show in the following screenshot:
4 - Do the same thing for the "Power" category, that is, verify that the "Enable this indicator on login" button at the bottom of that window is also enabled (dark), as I show in the following screenshot:
5 - Finally, check the "Session" category (which is the first category) and see if the options selected are right for you and, as for the other categories, verify that the "Enable this indicator on login" button at the bottom of that window is also enabled (dark), as I show in the following screenshot:
6 - Close that "Ayatana Indicator Settings" window. Finally, let's check that the "Ayatana Indicator Date & Time" is checked in the "Startup Programs" ( as @gordon nicely explains in another topic, but for the opposite issue of hiding that indicator - see How to disable Clock on the Indicator applet on MATE Panel? - #2 by gordon ). To do that, click again on the Ubuntu MATE Menu, hover your mouse over the "Preferences" menu to show its options, and then scroll down the right column until the "Startup Applications" is shown and then click on that "Startup Applications" icon in the right column), as I show in the following screenshot:
7 - That should have opened the "Startup Application Preferences" window. Check if the "Ayatana Indicator Application" option appears in the "Additional startup programs:" list and that it has the corresponding checkbox checked (if the corresponding checkbox isn't checked, then please check it) and do the same for the other "Ayatana Indicator ..." options that appear in that window ("Ayatana Indicator Date & Time", "Ayatana Indicator Keyboard", etc...). Here's the screenshot:
8 - Click the "Close" button to close the window.
9 - Log out and log in again.
Hopefully, the missing indicators will appear in the panel when you log in again.
I hope this helps
Thank you for taking the time to respond to my request. It pointed me in the right direction!
I opened the Preferences -> Indicators interface, but all the options there were grayed out, except for Notifications and Keyboard. However, the fact that it is named Ayatana Indicators allowed me to search from there. The name of the indicators pointed me to the following topic:
I searched for:
apt list --installed | grep ayatana
It turned out that the particular modules are missing. Then I searched for:
apt list | grep ayatana-indicator
It listed the available indicators. Some of them were listed as missing, but having a "residual config". This suggested that they were present at some point, but were uninstalled. I installed them with:
sudo apt install ayatana-indicator-power ayatana-indicator-printers ayatana-indicator-session ayatana-indicator-sound ayatana-indicator-messages ayatana-indicator-datetime
Then the indicators appeared upon log out/log in.
Thanks again for being so helpful!
You're welcome, @richterbg I'm very glad that my reply pointed you in the right direction and that you were able to solve the problem you were having of missing (Ayatana) indicators from the top bar. Thank you for having reported back with your further diagnostic process and for your great solution of installing missing "ayatana-indicator-*" software packages as it can be really useful to other users facing the same or similar problem.
