That means you've either made a syntax error in your configuration file, or you've deleted the file completely.
As for private server, do you mean a server ONLY for your local network, or an internet server only for you and your friend? If it's for local network then it'll work straight off, just connect using the server's a LAN address.
If it's for internet, then just set a password in the config file, and you'll need to forward ports through your router too. Have a look at http://www.portforward.com for a guide to doing that.