My first install of Ubuntu was 16.04 with the only problems being with Ubuntu not wanting to play nice with my Nvidia graphics card. I later upgraded to 17.10 with no problems. I just now upgraded to 18.04. All appeared to go fine until the upgrade process finished and it asked to restart. I said okay. The system then rebooted to a blank purple screen.
If I forcibly power down and then turn power on I get to a grub menu. I can get in if I select an alternate boot option but when I shut down, the next time I boot up I'm right back at the blank purple screen.
On one of these alternate boots, I went in to XDiagnostics and turned on the options for extra debug logging and turned off the bootloader graphics. I can now see that Ubuntu apparently goes into an infinite hang on "a start job is running for detect the available gpus and deal with any system changes"
Is there a definitive fix for this?
If there is not a fix for this then how can I downgrade the install (without loss of data) to 17.10 which worked just fine?