I upgraded my kde neon from 18.04 to 20.04 using this Tutorial. (The terminal Version). After final reboot, i logged in (the splash screen was taking much longer than usually) And there was no Desktop and no task bar, just a black screen. There was also my thunderbird showing up, but in white (I've set the theme breeze dark, which seems not to apply to the thunderbird after upgrade). what still works is alt+f2 to launch apps. I think it's worth mentioning that i did the system upgrade in the terminal because the GUI claimed I should do all updates even though I updated my system via discover or pkcon litrally 1 minute before.