su: incorrect password

After a long time of stable work of a Linux server (RHEL 5) it suddenly started to refuse logins under regular users. Only root logins were allowed. I guess, it's because of password expiration of my user (the only interactive non-root user in the system) or after some system update which has been done by another adminstrator.

First of all I tried to reset password of my user under root "passwd <user>", but it didn't do the trick. What was strange is that su didn't prompt for password and just returned "su: incorrect password"

There were a lot of reasons of such a problem which were described on a lot of forums. The only solution that helped me has been shown by dballagh in this thread:
pam_tally --user <user> --reset
My user has been blocked by pam_tally module after password expiration or a lot of failed logins.