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There were updates offered by Ubuntu and I installed them. After that the Ubuntu desktop GUI hasn't been working correctly and it freezes frequently. Moreover, instead of having the Ubuntu startup screen like below, only a blank gray screen comes, then on the desktop instead of wallpaper, only a black background is applied.

enter image description here

I also tried this sudo apt-get install ubuntu-desktop but get thee results shown below. 

The following packages have unmet dependencies:
 ubuntu-desktop : Depends: checkbox-gui but it is not going to be installed
                  Recommends: python3-aptdaemon.pkcompat but it is not going to be installed
                  Recommends: unity-webapps-common but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

I tried to install checkbox-gui, then it gives another unmet dependencies error message, than another then another.

here is another screenshot of the Ubuntu desktop right now, enter image description here

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  • If you're using a laptop/desktop that comes shipped with Ubuntu and you haven't added another OS for dual-booting, you may be out of luck. But, if you're using a dual-boot system or virtual machine on a Windows/OS X/Other Linux distro as the host OS, and if you're willing to wipe your Ubuntu data, then you might be able to create a new virtual machine or reinstall Ubuntu itself.
    – giikae
    Jan 16, 2017 at 6:26

2 Answers 2

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TL;DR:

Always remove PPAs with ppa-purge

Whole story:

Yesterday I wanted to try KDE Plasma and I installed it with following commands:

sudo add-apt-repository ppa:kubuntu-ppa/backports
sudo apt-get update && sudo apt-get dist-upgrade
sudo apt-get install kubuntu-desktop

After hour I decided to switch back to Unity and remove KDE:

sudo add-apt-repository --remove ppa:kubuntu-ppa/backports
sudo apt-get update && sudo apt-get dist-upgrade
sudo apt-get remove --purge kubuntu-desktop

After login I was facing few problems, so I tried reinstalling ubuntu-desktop and unity, but I was getting error:

The following packages have unmet dependencies:
 ubuntu-desktop : Depends: checkbox-gui but it is not going to be installed
                  Recommends: python3-aptdaemon.pkcompat but it is not going to be installed
                  Recommends: unity-webapps-common but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

Some digging and I found out what cause it: removing PPA does not downgrade packages. The solution was to add it again and execute:

sudo apt-get install ppa-purge
sudo ppa-purge ppa:kubuntu-ppa/backports

And then I was able to reinstall ubuntu-desktop :)

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  • Your answer matched my situation, but I don't understand how did you find that we should downgrade the packages. Could you explain further?
    – zsf222
    Oct 24, 2018 at 2:39
  • @zsf222 I am not sure, but I probably searched for keywords like "ubuntu", "ppa", "unmet dependencies" and found information that manual PPA removal is not safe, because packages are not downgraded.
    – andrzej1_1
    Oct 24, 2018 at 11:46
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Below commands solve the my issue, unity --replace sudo apt-get install --reinstall unity sudo dpkg --configure -a
sudo apt-get update
sudo apt-get upgrade -y

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