. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified) (.Net SqlClient Data Provider)
//. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified) (.Net SqlClient Data Provider)
Goto window+R and type services.msc and press enter.
Then start SQL server manually if not automatically started.
Then you try to login, it must be works.