Problem booting after updating from Ubuntu 14.10 to 15.04

I was able to fix this after reading the bug report @Mondane mentioned and several other troubleshooting articles on recovery mode.

1. Boot into GRUB menu (pressing shift while starting).
2. Choose recovery mode.
3. Enable networking.
4. Drop to root shell prompt.
5. ifconfig eth0 and dhclient eth0 to get
6. (Probably unnecessary) I had a some networking issues that were probably unrelated to this (undetermined so far) so I had to edit /etc/resolv.conf to include Google DNS 8.8.8.8 and 8.8.4.4.
7. See what drivers are available for the nvidia hardware: ubuntu-drivers devices
8. Install the recommended: apt-get install nvidia-346
9. Reboot normally.
10. Go to sleep.

Some resources I found useful:

• https://wiki.ubuntu.com/RecoveryMode#Booting_into_recovery_mode
• How do I boot into a root shell?
• https://help.ubuntu.com/community/BinaryDriverHowto/Nvidia

For reasons I haven't yet determined, my networking was really messed up. I had to unplug and replub the USB-to-ethernet, run dhclient, then apt-get install nvidia-346 to get about 5% of the update before networking failed. This might be a hardware issue and totally unrelated, but it made troubleshooting and fixing this problem especially difficult.