Ubuntu 16.04 had been working well for over a year. Then, after a standard software update, as it was very slow I shut down and tried to re-boot.
I was led first to the blank purple screen. Eventually I got the menu to select recovery linux versions. I tried Linux 4.15.0-39-generic and that tried to boot, getting to a black and white screen ending with ---[end Kernel panic - not syncing: VFS: unable to mount root fs on unknown block(0,0)
I tried getting to the Grub prompt but had no success trying to boot manually, using the sugggestions in thread After update in Ubuntu 16.04 I ended up in GNU GRUB and appear to be stuck there