I upgraded from 13.10 to 14.04, and when I restarted the computer and selected Ubuntu, the black screen said:
mount: mounting/dev/loop0 on/root failed: Invalid Argument
mount: mounting/dev on/root/dev failed: No Such File or Directory
mount: mounting/sys on/root/sys failed: No Such File or Directory
mount: mounting/proc on/root/proc failed: No Such File or Directory
Target Filesystem doesn't have requested /sbin/init.
No init found. Try passing init=bootarg.
BusyBox v.1.21.1 (Ubuntu1:1.21.0-1ubuntu1)built-in shell(ash)
Enter 'help' for a list of built-in commands.
When I select Ubuntu from the boot manager, I hold the shift key
This brings up Grub manager
With the first line selected, hit "e"
Hunt down through the boot instructions to a long command line that includes options "ro quiet splash"
Change the "ro" to "rw" (readonly to read-write)
ctl-x to save and run
This brought me right up in 14.04 LTS. The disclaimer is that I don't have any idea why it worked. I then edited /boot/grub.cfg to make this same change permanent (suggestions on the correct way to do this appreciated).