Sometimes after some weeks of uptime when I entered my office in the morning, I discovered a freezed Plasma. To fix this, enter the system via SSH and then execute this command:

sudo service sddm stop

And without re-login:

kquitapp5 plasmashell && kstart5 plasmashell

If you got any error of the following when running apt or anything else:

locale: Cannot set LC_ALL to default locale: No such file or directory

perl: warning: Falling back to the standard locale (“C”)

perl: warning: Setting locale failed.

Just run the following command and select the correct locale:

sudo dpkg-reconfigure locales

Or use:

sudo localectl set-locale en_US.UTF-8