Several months after I have installed kde plasma 6, everytime after zorin booting up (after zorin logo animation) most freqently shows a blank black screen and sometimes this (It could be becauce I have full of storage)
You have installed Plasma 6 on Zorin? But not from the official Repo's, right? Because there Plasma 6 isn't available.
The Error show a faild Service started. What Login Screen do You use? The Plasma one or the Zorin one?
For this, You could start a Bootstick (with whatever System) and run it in Live Mode. then open the File Manager, go to ''Other Places'' or choose the Drive on the left Side where Zorin is installed and then click through the Folders until You come to the Files, You want save.