I think the first step to troubleshooting this is knowing what error you get. Is it that it doesn't like your username/password or is it a TCP/IP error or a timeout or something else?
Are you connecting from the same machine as the SQL instance or from a remote machine? I ask this because it could be firewall issues.
Did the services start? If the services are not running (just the SQL Server service, the browser service is not required but I recommend it, the agent service is not required but is recommended too), you will need to start those up.