I usually log into my server via SSH as a non-root user and then authenticate myself as root using the su command whenever I need it.
The thing is I haven’t changed a thing (not configurations, passwords, installations, etc.) and since last Saturday I get an “Authentication Failure” message whenever I try to su.
I’ve seen some solutions to this problem but most of them require access to editing files and issuing commands that my current user has no privileges for.

What can I do to solve this issue?