Cannot boot to a graphical desktop, black screen before login prompt.

[David Smith]

Upgraded from 18.04 to 18.10, did not boot afterwards.

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: lightdm 1.28.0-0ubuntu1
ProcVersionSignature: Ubuntu 4.18.0-10.11-generic 4.18.12
Uname: Linux 4.18.0-10-generic x86_64
ApportVersion: 2.20.10-0ubuntu13
Architecture: amd64
Date: Sat Oct 20 11:38:43 2018
InstallationDate: Installed on 2018-09-20 (29 days ago)
InstallationMedia: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
ProcEnviron:
 LANGUAGE=en_GB:en
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: lightdm
UpgradeStatus: Upgraded to cosmic on 2018-10-19 (0 days ago)