Can not su to root anymore

Suddenly my ability to su to root (su root, or su -)
has ceased.  No message, just returns my to a prompt
and I remain in my current login shell. securettys
still has /dev/comsole, no /etc/default/login file,
and both my oginal account and root are in the system
group.

Thanks for the help.

5.1A PK6

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