This has happen on my Dell laptop much more than I’d like:
http://bernaerts.dyndns.org/linux/232-ubuntu-boot-failure-initramfs
mount: mounting /dev on /root/dev failed: No such file or directory
mount: mounting /proc on /root/proc failed: No such file or directory
Target filesystem doesn’t have /sbin/init.
No init found. Try passing init=bootarg.
BusyBox v1.18.5 (Ubuntu 1:1.18.5-1ubuntu4) built-in shell (ash)
Enter ‘help’ for a list of built-in commands.
(initramfs)