I’ve been unable to connect to a server via one of its aliases for about a week.  When I looked at the TCP/IP Properties for the instance I saw the following:

Note that the Active flag for IP1 is “No”. BOL defines the Active flag as “Indicates that the IP address is active on the computer.” However I know that the IP is active since it is working for another instance on the same server.
I tried changing the Active flag to “Yes”, then applying the changes. When I brought back up the page it still showed “No”. Next I restarted the instance, no change. Then I restarted the server, still no luck.
Eventually I checked the registry. They key for IP1 is: [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\MSSQL10.SQL2008\MSSQLServer\SuperSocketNetLib\Tcp\IP1]
(The part MSSQL10.SQL2008 represents the instance. Change it to match the instance you are checking.)

Under this key is an entry “Active” that is 0x00000001 (1) for “Yes” and 0x00000000 (0) for “No”. I first changed the flag from 0 to 1. When I went back to SSCM the TCP/IP Properties page now showed:

Once I restarted the instance I could connect normally!

I’m a little surprised that SQL doesn’t update this flag without a registry change and I wouldn’t recommend it normally, but in this case it seems necessary.