KDE Neon Black Login Screen
You are Welcome! The Tutorial Shows You Step-by-Step How to Recover KDE Neon Black Screen on Boot Issue 32/64-bit GNU Desktops.
Especially revevant: this Behavior may be provoked from Errors on the Boot Device.
So try first to Check and Fix Each ext2/ext3/ext4 Partition present on the Hard Disk.
1. Booting Live
Boot System with KDE Neon or other Ubuntu/Debian Live Media.