Problem to access the SQL Server
Make sure SQL Server Service is running
If a named instance, make sure SQL Server browser service is running
Make sure SQL Server is configured to allow remote connections
Examine the SQL Server error log for messages confirming that SQL is listening on the  expected network interfaces and ports
Test server connectivity with PING from the client machine
Test port connectivity using TELNET or PowerShell to the server and port (from step 4) from the client machine.  For example

a. TELNET  1433

b. PowerShell: 


1433 | % { echo ((new-object Net.Sockets.TcpClient).Connect("YourServerName",$_)) "server listening on TCP port $_" }


Check firewall settings if step 5 or 6 connectivity test fails


For further read below forum discussing the same which would be helpful to you for troubleshooting :

https://stackoverflow.com/questions/18060667/cannot-connect-to-server-a-network-related-or-instance-specific-error