Check your PostgreSQL pg_hba.conf. 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