If you are connecting through Sql Sever Mgmt Studio, make sure that you put the port number after the IP address. (i.e. xxx.xxx.xx.xx:1433) 1433 typically is the open port, but I usually change mine. If you change it (port #) on your router, then make sure you change it in SQL server. Also make sure that SSMS is using Mixed Mode authentication, not just Windows.

Hope this helps