I recently changed my password using mkpasswd. Then I stepped away from the computer and my screensaver went on so I put in my new password. It didn't work. I tried my old password and it didn't work. I switched to a terminal logged in as root, changed my password to username went back to the screensaver and it still didn't work. Then I restarted X and tried to login and it keeps giving me Authentication Failure. I'm sure the password I'm putting in is correct.
Also, if this has anything to do with it, I'm trying to get a passwordless login to work.

did you use passwd to change the password?

I was using root to do the password change so I didn't think of using passwd. (note that the mkpasswd man page says that it first uses yppasswd then passwd to assign passwords to a user).
I tried it and I found out that both of these programs give me an error:

yppasswd: can't get local yp domain: local domain name not set
passwd: Authentication token manipulation error