Tell you what.
If youknow howto, switch to a viirtual tyerminal
ctrl+alt+f3 is a good choice.
login as a regular user and follow to nest set of commands.
apt-get -y remove kde && apt-get -y install kde && apt-get -y update && apt-get -y upgrade kde
Follow the install and choose whichever display manager yoy want plus any other configurations. You can skip samba and other workgroups if they are not important.
After the commands are finished type exit twice until the session is closed.
Use alt+ctrl+f7 to return to the login screen and reboot the machine.
It should be fixed.