Hi Keith, and welcome to the forum! I hope that you've got it resolved and you are able to access your system now. I have long had trouble making the trick work to "hold down the shift key" in order to access the GRUB menu, and I've discovered that some computers (mine!) need to tap the ESC key instead.... just tapping it one time only after the BIOS splash. If I tap ESC twice it goes past the GRUB menu and leaves me at a grub> prompt (not where I want to be).
With the single tap of the ESC key, I get the standard GRUB menu and first select "Advanced options for Ubuntu"... and then Ubuntu (recovery mode). This all worked for me with the latest Ubuntu 18.04 just now, but your system may indeed work with the shift key instead. Good luck!