I've run a dual boot Windows 10/Ubuntu 16.04 LTS laptop for years. This morning I had it over on the Windows side and booted to Ubuntu (where I spend most of my time).
I log on and run updates while I have everything closed. Leave it to sit for a bit and comeback and it's frozen. Reboot and it reboots to the purple standard logon screen. No GUI launches though the background is mine.
Went to ctr+alt+f1, and it says 'Welcome to Ubuntu 18.04, not 16.04 like the logon screen said. It was definitely the 16.04 background. The computer can't find anything lightdm, or gdm. I also ran apt-get upgrade again hoping that it had interrupted and it didn't help.
I'm sorry, I don't know how I can paste code, since I'm back over to Windows to post this.