The main cause is that the server instance is missing cannot be found on the network. Possible Cause/Solution:
1.) Server Computer (PC where SQL Server Instance was installed) might habe been turn off.
2.) Server Computer or Client PC currently doesn't have network connection -> You can check the connection by pinging two PC to each other. Refer to this link: http://www.wikihow.com/Ping-an-IP-Address
3.) SQL Server Instance has stopped. Refer to this link: http://msdn.microsoft.com/en-us/library/hh403394.aspx
4.) SQL Server Instance was blocked by Windows Firewall. Other Possible Solution: http://support.qne.com.my/article/3203/60584/cannot-login-to-qne-optimum-system-prompted-named-pipes-provider-error-40-could-not-open-a-connection-to-sql-server Please inform us if this response helped you. Thanks, Eddnie