problems with authentication

Once again, I am in need of some assistance.

I have been handed a postgresql 7.2.1 db running on an AIX platform that
is suddenly having authentication errors.  We are not sure what has
happened, but starting last Friday we started having the following in
the log files.

Password is stored MD5 encrypted.  'password' and 'crypt' auth methods
cannot be used.
FATAL 1:  Password authentication failed for user "<user-name>"

Can anyone direct me on what/where to look for fixing this issue?  I am
not familiar with this version of Postgresql or with it running on AIX.

thanks for any help.

Chris