What are you connecting with and what is the error you're getting? Are you trying to use SQL Server Management Studio?
Make sure the SQL Browser Service is running on the server
Make sure TCP/IP is enabled on your client's SQL Native Client configuration.
1. Open SQL Server Configuration Manager
2. Expand SQL Native Client Configuration
3. Click Client Protocols, make sure TCP/IP is enabled
Also, make sure you've got the right port(s) open on the firewall (I know you said you already have the port open). Default is TCP/1433. This can be changed, though, and if you're using a named instance you will probably need to assign it a static port and add it to the firewall config.