.net链接SQL时遇到“登录过程中出错，虽然已经成功建立了与服务器的连接。错误信息：(TCP提供商，错误代码0 - 指定的网络名称不可用)”

[SqlException (0x80131904): 已成功与服务器建立连接，但是在登录过程中发生错误。 (provider: TCP 提供程序, error: 0 - 指定的网络名不再可用。)] System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection) +171 System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj) +199 System.Data.SqlClient.TdsParserStateObject.ReadSniError(TdsParserStateObject stateObj, UInt32 error) +529 System.Data.SqlClient.TdsParserStateObject.ReadSni(DbAsyncResult asyncResult, TdsParserStateObject stateObj) +157 System.Data.SqlClient.TdsParserStateObject.ReadPacket(Int32 bytesExpected) +79 System.Data.SqlClient.TdsParserStateObject.ReadBuffer() +44 System.Data.SqlClient.TdsParserStateObject.ReadByte() +18 System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj) +74 System.Data.SqlClient.SqlInternalConnectionTds.CompleteLogin(Boolean enlistOK) +34 System.Data.SqlClient.SqlInternalConnectionTds.OpenLoginEnlist(SqlConnection owningObject, SqlConnectionString connectionOptions, String newPassword, Boolean redirectedUserInstance) +606 System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, Object providerInfo, String newPassword, SqlConnection owningObject, Boolean redirectedUserInstance) +193 System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection) +502 System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnection owningConnection, DbConnectionPool pool, DbConnectionOptions options) +28 System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject) +429 System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject) +70 System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject) +512 System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection) +85 System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory) +89 System.Data.SqlClient.SqlConnection.Open() +160 Microsoft.Practices.EnterpriseLibrary.Data.Database.OpenConnection() +196 Microsoft.Practices.EnterpriseLibrary.Data.Database.ExecuteReader(DbCommand command) +53 HZZZ.DAL.Sys.SysUser.IsExists(String userName, String password) in E:\Works\惠州\Source\HZZZZ\HZZZ.DAL\Sys\SysUser.cs:347 HZZZ.BLL.Sys.SysUser.IsExists(String userName, String password) in E:\Works\惠州\Source\HZZZZ\HZZZ.BLL\Sys\SysUser.cs:191 Logon.IsAuthenticated() in e:\Works\惠州\Source\HZZZZ\HZZZ.WebUI\Logon.aspx.cs:40 Logon.btnSubmit_Click(Object sender, EventArgs e) in e:\Works\惠州\Source\HZZZZ\HZZZ.WebUI\Logon.aspx.cs:25 System.Web.UI.WebControls.Button.OnClick(EventArgs e) +75 System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) +97 System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +7 System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +11 System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +33 System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +4919