Welcome to LQ!
A common cause of what you describe is a system booting with a -generic kernel without a suitable supporting initrd (initial ramdisk).
As you have found, you can use the install media to access the system. You can use this to 'chroot' to your install.
Then you can use the '/usr/share/mkinitrd/mkinitrd_command_generator.sh' script to generate a suitable 'mkinitrd' command, run it and adjust your bootloader.
Further information on
chroot and
initrd.