Windows Security / Enter network credentials

Question

Today, all the office Windows workstations were logged in to the AD/DC Administrator to update a printer driver, then the admin logged out and the users logged back in with their own credentials. That worked fine, except for two users. After entering their credential a window pops up asking the user to "Enter network credentials", and it shows the user's email address (not their domain ID) and a space for a password. No password works. I've never seen this dialog before and am at a loss as to what to do. These users cannot log into their workstations.