Why do I get the error message ‘Sql Server does not exists or access denied ‘ whenever I try to access the data on Sql server

• The ASP.NET worker process doesn’t have the correct permissions to connect to or write to the SQL Server database. Either enable impersonation for users, or configure the ASP.NET worker process to run under the SYSTEM account.
• The ASPNET account cannot establish a named pipes connection to the database server. Try using TCP/IP instead of the Named Pipes protocol to connect to the database by adding ‘Network Library =dbmssocn’ in your connection string

For more information refer

• BUG: Named pipes do not work when worker process runs under ASPNET account
• Process and request identity in ASP.NET