Should provide a postgresql prompt. If it doesn’t and complains about a missing role as well, then it most likely is a missing user you have or you accidently removed your database. Don’t know.
Hard to tell with the provided information.
A mere restart wouldn’t cause this. What have you done before restarting?
You could try manually re-adding the pgsql user with createdb zammad -O zammad as postgres user.
Apart from that I’m afraid I won’t be able to help you.