Not sure what happened but now when I try to boot in to elementary OS I get a KDE plasma (I think?) loading screen and then an error. I was using my laptop at work and hadn't changed anything. I get home and boot it up and now this is happening.
Most probably a KDE package including desktop was installed on that system but that is not working normally.
It also seems that (however faulty) the KDE desktop is the one selected first.
If your elementary desktop and the lightdm greater are still in place:
You should try to boot into the elementary desktop. When more desktop environments are installed you should see a gears button at the login screen (upper-right of the password field).
Go to the login screen and see if that button is available.
If you cannot logout and/or you cannot get to the login screen:
Hit ctrl+alt+F1 to go to a tty prompt
you may need to enter username and password; enter
sudo killall lightdm- that should restart the login greater; to start it manually enter
If you want to keep both desktop environments, you can stay with Cipricius's answer.
If there is a KDE desktop installed and you don't need it you can do in a terminal window
sudo apt install elementary-desktop plasma-desktop-
This latter will (re)install the elementary OS Pantheon desktop and remove the KDE Plasma desktop. This can fix some glitches from software libraries incompatibles with elementary OS (display inconsistencies, etc.)