16.04 upgrade won't start

Did the upgrade succeed? Are you now using 16.04? Is it working well?

Linux prints system messages to the screen during the loading of Linux. Most of the time this printing of system messages to the screen is suppressed and hidden but sometimes we see a message or two. They are not necessarily critical errors.

If this error is preventing Linux & Ubuntu from loading, then you can try going to Advance options for Ubuntu at the Grub boot menu and in the sub-menu select a Linux keel with recovery mode.

At the recovery menu select Network to get an inteet coection and then select Root shell prompt and run

apt update
apt upgrade
apt full-upgrade

Regards.