Sometimes while trying to establish connection to the database the following error message will come
This error is coming because the server name is incorrect. Or Sql server is not running on the server.
To resolve the issue follow the below steps
1. Start -> Sql Server
2. In file -> Connect Object Exporer
3. Copy the server name and put into data source
4. If not working use system name. If Sqlexpress add SQLEXPRESS also
5. Make sure sql server is running