Is the TCP/IP protocol enabled for SQL connections?

To enable TCP/IP the run the SQL Server Configuration Manager (on the SQL Database Server).

In later version of SQL you may need to run the mmc.exe. 

File menu of an open MMC 3.0 console, click Add/Remove Snap-in

Find the SQL Server Configuration Manager and Add>, press OK

Open options for SQL Server Network configuration> Protocols for [my SQL server]

Make sure TCP/IP is set to ENABLED