It is possible that you altered the 'postgres' user which is vital for PostgreSQL to work. Without knowing what you did I'm not sure how to reverse the changes, but you could uninstall and reinstall PostgreSQL and that will recreate the postgres user without removing your databases.
Follow this guide
to uninstall and reinstall. Note that to reconnect to your existing databases you will need to reinstall the same major version that you currently have (from your error message that is 8.3).