I was having issues with plasma freezing on me so I went into the video settings and removed the nvidia drivers. When I did that my comp went to reboot automatically and now it is just coming up with the journal scan message but no desktop.
have you tried to access the terminal with pressed CTRL+ALT-F2 ?
If you can login there you can try to reinstall nvidia drivers or remove the blacklist for the nouveau-drivers in /etc/modprobe.d/mhwd-gpu.conf so that your system works with the free drivers.