Actually, though, it sounds like something broke with sudo. You'll probably have to get physical access to the machine and boot it up in single-user mode to modify the proper files without sudo access, but if you have a root account already enabled, you can maybe SSH into the machine and poke around there.
Once you get into the machine, hop into /var/log and then run ```
grep pam *.log
``` to find the log that contains the PAM error, which should tell you which configuration file is missing or corrupt. Otherwise, you might be able to cd into /etc/pam.d/ and see if any of these files are missing from the machine:
chkpasswd login passwd screensaver sudo
cups login.term rshd sshd
ftpd other samba su
Since you're seeing the authentication module error with sudo, you might be able to just create the ASCII text file from scratch. Below is what the file contains on my machine, so if you copy that direct, that could do the trick. Otherwise, if the logs reveal it's a different configuration that's causing issues, you can find what the text looks like on a good machine and copy it over from there.
# sudo: auth account password session
auth required pam_opendirectory.so
account required pam_permit.so
password required pam_deny.so
session required pam_permit.so
Hope this helps!
