Following are the three most common issues.
Issue #1: Sometimes the SQL Server Browser Service has not been started. This is needed for your SQL machine to broadcast to the network. To verify this, right-click My Computer. Then select Manage>Services>SQL Server Browser. Select the service and click the Play button on the toolbar.
Issue #2: Windows Firewall by default will block SQL ports to the outside world. The easiest solution is to turn the firewall off, but that is the more unsecure method. The other method would be to set an exception for SQL and the SQL Server Browser in the Firewall configuration.
Default SQL port: TCP/IP 1433
Default SQL Server Browser port: UDP 1434
Issue #3: Named Pipes or TCP/IP (or both) have been disabled in the SQL Configuration. Open the SQL Server Configuration Manager. Under SQL Server network configuration, select Protocols for SQLEXPRESS (MSSQLServer will be shown if you have the standard edition, or if a custom instance name was used during SQL installation, that will be shown here). Verify Named Pipes or TCP/IP is set to enabled. You may try setting both to enabled.