I had to force shutdown (by long pressing the start button) Ubuntu 18.04 after the system froze. In such cases usually I would try grub boot loader. A previous solution suggested steps from grub menu (Error: invalid environment block. Press any key to continue), but in my case even the grub menu is not showing.
Boot a live environment (eg. your Ubuntu install media, selecting 'try ubuntu') and do a
fsck (file system check) of your partitions.
Reboot and it should be okay.
This is suggested due to a scan of https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/439784 (old) which showed the most common cause for this error was file-system corruption.