C# 用SQLDMO备份还原数据库问题

private void 备份ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            SQLDMO.Backup oBackup = new SQLDMO.BackupClass();
            SQLDMO.SQLServer oSQLServer = new SQLDMO.SQLServerClass();
            try
            {
                oSQLServer.LoginSecure = false;
                oSQLServer.Connect(Dns.GetHostName(), "sa", "123456");
                oBackup.Action = SQLDMO.SQLDMO_BACKUP_TYPE.SQLDMOBackup_Database;
                oBackup.Database = "HORIZON";
                oBackup.Files = @"d:\HORIZON.bak";
                oBackup.BackupSetName = "HORIZON";
                oBackup.BackupSetDescription = "数据库备份";
                oBackup.Initialize = true;
                oBackup.SQLBackup(oSQLServer);
            }
            catch
            {
                throw;
            }
            finally
            {
                oSQLServer.DisConnect();
            }         }
[Microsoft][ODBC SQL Server Driver][Shared Memory]SQL Server 不存在或访问被拒绝
[Microsoft][ODBC SQL Server Driver][Shared Memory]ConnectionOpen (Connect()).请高手帮小弟解答下  为什么错误了