Problem: I must have upgraded something during today and at some point I could no longer sudo. I rebooted and couldn't log in. Checking /etc/shadow showed all accounts were locked with ! passwords.

Workaround: I rebooted into the NixOS live CD, mounted the partition, removed the 'x' from /etc/passwd for root (to disable pass encryption), found the passwd binary in the mounted Nix store and changed the password for root. This let me log in and change my user account.

Configuration: I had users.mutableUsers = false which I think was recently changed (I've now flipped this in my config, to ensure this doesn't happen in the future) and I'm using NixOS unstable which I built some time today (11/11/2014)