Named instances listen on a non standard port, either dynamic or static.
The SQL Server Browser service tells the clients on which port the named instance is listening. If you cannot contact the UDP port 1434, you can't connect to the browser service and you don't know which port to attach to. First thing I would check is firewall rules.
If you can connect to the named instance without specifying the instance name, two things could have happened:
1) There's an alias in your native client configuration
2) Somebody changed the port of the named instance to TCP 1433.
Hope this helps
Gianluca