Linux Mint 18.1 Serena to 18.2 Sonya
 

Upgraded my laptop first and upon reboot, it asked for a user password I'd never created. Unable to login as root as well, so I had to simply reinstall 18.1. On my desktop machine at home, I upgraded and the same thing happened. I make a monthly backup, so I simply did a restore and only lost a few pictures. WHAT GIVES!?!?!

Normally, a root and one user is created. Both have passwords.

Normally, a root and one user is created. Both have passwords.
 

I wasn't asked to create anything. I have the root account which has a password and my user account which I didn't password protect. This was in 18.1(serena) When I upgraded to 18.2 (sonya) I wasn't asked to add/change anything. It finished and prompted me to reboot. I did and then wasn't able to log in. My user account in 18.1 had no password and I was unable to login using the root account either. If during the upgrade process, I was prompted to create a new user and root account, I would've been in the clear. Because I "assumed" the upgrade would use the user and root accounts from the previous version, I didn't question it. LAST TIME I do that!!!

You could have just pressed e during grub boot (press and hold shift if you do not see that option) to enter grub config and add init=/bin/sh to the end of the linux... line entry, save with cntl-x to get a one time console and use passwd to set/reset your root password. Or your regular user account pwd and use sudo after rebooting.