A network-related or instance-specific error occurred while establishing .... error: 26

I am working in my asp.net project; when I run my program, it stops running and gives me this message:

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified) Server Error in '/Test2' Application.

A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.Data.SqlClient.SqlException: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)

Source Error:


Line 27:         string cmdString = "Delete from tblSessionCart";
Line 28:         SqlCommand cmd = new SqlCommand(cmdString, conn);
Line 29:         conn.Open();
Line 30:         try
Line 31:         {


Source File: c:\Users\mousa\Desktop\Test2\App_Code\clsSessionCart.cs    Line: 29

Stack Trace:

[SqlException (0x80131904): A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified)]
   System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection) +4876455
   System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj) +194
   System.Data.SqlClient.TdsParser.Connect(ServerInfo serverInfo, SqlInternalConnectionTds connHandler, Boolean ignoreSniOpenTimeout, Int64 timerExpire, Boolean encrypt, Boolean trustServerCert, Boolean integratedSecurity, SqlConnection owningObject) +354
   System.Data.SqlClient.SqlInternalConnectionTds.AttemptOneLogin(ServerInfo serverInfo, String newPassword, Boolean ignoreSniOpenTimeout, Int64 timerExpire, SqlConnection owningObject) +90
   System.Data.SqlClient.SqlInternalConnectionTds.LoginNoFailover(String host, String newPassword, Boolean redirectedUserInstance, SqlConnection owningObject, SqlConnectionString connectionOptions, Int64 timerStart) +401
   System.Data.SqlClient.SqlInternalConnectionTds.OpenLoginEnlist(SqlConnection owningObject, SqlConnectionString connectionOptions, String newPassword, Boolean redirectedUserInstance) +225
   System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, Object providerInfo, String newPassword, SqlConnection owningObject, Boolean redirectedUserInstance) +189
   System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection) +4889331
   System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnection owningConnection, DbConnectionPool pool, DbConnectionOptions options) +31
   System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject) +431
   System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject) +66
   System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject) +499
   System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection) +65
   System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory) +117
   System.Data.SqlClient.SqlConnection.Open() +122
   clsSessionCart.DeleteCart() in c:\Users\mousa\Desktop\Test2\App_Code\clsSessionCart.cs:29
   ASP.global_asax.Session_Start(Object sender, EventArgs e) in c:\Users\mousa\Desktop\Test2\Global.asax:27
   System.Web.SessionState.SessionStateModule.RaiseOnStart(EventArgs e) +8878884
   System.Web.SessionState.SessionStateModule.CompleteAcquireState() +237
   System.Web.SessionState.SessionStateModule.BeginAcquireState(Object source, EventArgs e, AsyncCallback cb, Object extraData) +504
   System.Web.AsyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +66
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155


Version Information: Microsoft .NET Framework Version:2.0.50727.5477; ASP.NET Version:2.0.50727.5479 

This problem is related to connection string,

1. Check instance server
2. Check user and password

Connection string:

Server=YOUR_SQLSERVER_INSTANCE;Database=YOUR_DATABASE_NAME;User Id=sa;Password=YOUR_SA_PASSWORD;

If you have instance, make sure instance is specified in server, for example:

Server=.\SQL2008