Check your PostgreSQL [`pg_hba.conf`](https://www.postgresql.org/docs/current/static/auth-pg-hba-conf.html). It should have lines like this **copied directly from the docs**.

    # Allow any user on the local system to connect to any database with
    # any database user name using Unix-domain sockets (the default for local
    # connections).
    #
    # TYPE  DATABASE        USER            ADDRESS                 METHOD
    local   all             all                                     trust
    
    # The same using local loopback TCP/IP connections.
    #
    # TYPE  DATABASE        USER            ADDRESS                 METHOD
    host    all             all             127.0.0.1/32            trust

You don't have to keep these. But it's normal to have these so you can connect locally without having to worry about passwords using your local system authentication methods. This will allow you to log in s any user.

So you can log in as PostgreSQL with,

    sudo -u postgresql psql -d myDatabase