Login failed for user 'x' in SQL Server

We received an error "Login failed for user 'x'" " sometime from application. We are 100 % sure as it working when we checked by manual login from SSMS as well as from application. It happened sometime only. How can we found exact reason for it's failure? why it failed sometime? Below is a whole stack trace.

Exception Type : System.Data.SqlClient.SqlException

Message : Login failed for user 'x'.

Source : .Net SqlClient Data Provider

TargetSite : Void OnError(System.Data.SqlClient.SqlException, Boolean)

StackTrace :

at System.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection) at System.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj) at System.Data.SqlClient.TdsParser.Run(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj) at System.Data.SqlClient.SqlInternalConnectionTds.CompleteLogin(Boolean enlistOK) at System.Data.SqlClient.SqlInternalConnectionTds.AttemptOneLogin(ServerInfo serverInfo, String newPassword, Boolean ignoreSniOpenTimeout, Int64 timerExpire, SqlConnection owningObject, Boolean withFailover) at System.Data.SqlClient.SqlInternalConnectionTds.LoginWithFailover(Boolean useFailoverHost, String primaryHost, String failoverHost, String newPassword, Boolean redirectedUserInstance, SqlConnection owningObject, SqlConnectionString connectionOptions, Int64 timerStart) at System.Data.SqlClient.SqlInternalConnectionTds.OpenLoginEnlist(SqlConnection owningObject, SqlConnectionString connectionOptions, String newPassword, Boolean redirectedUserInstance) at System.Data.SqlClient.SqlInternalConnectionTds..ctor(DbConnectionPoolIdentity identity, SqlConnectionString connectionOptions, Object providerInfo, String newPassword, SqlConnection owningObject, Boolean redirectedUserInstance) at System.Data.SqlClient.SqlConnectionFactory.CreateConnection(DbConnectionOptions options, Object poolGroupProviderInfo, DbConnectionPool pool, DbConnection owningConnection) at System.Data.ProviderBase.DbConnectionFactory.CreatePooledConnection(DbConnection owningConnection, DbConnectionPool pool, DbConnectionOptions options) at System.Data.ProviderBase.DbConnectionPool.CreateObject(DbConnection owningObject) at System.Data.ProviderBase.DbConnectionPool.UserCreateRequest(DbConnection owningObject) at System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject) at System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection) at System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory) at System.Data.SqlClient.SqlConnection.Open() at System.Data.Common.DbDataAdapter.QuietOpen(IDbConnection connection, ConnectionState& originalState) at System.Data.Common.DbDataAdapter.FillInternal(DataSet dataset, DataTable[] datatables, Int32 startRecord, Int32 maxRecords, String srcTable, IDbCommand command, CommandBehavior behavior) at System.Data.Common.DbDataAdapter.Fill(DataSet dataSet, Int32 startRecord, Int32 maxRecords, String srcTable, IDbCommand command, CommandBehavior behavior) at System.Data.Common.DbDataAdapter.Fill(DataSet dataSet) at Microsoft.Practices.EnterpriseLibrary.Data.Database.DoLoadDataSet(DbCommand command, DataSet dataSet, String[] tableNames) at Microsoft.Practices.EnterpriseLibrary.Data.Database.LoadDataSet(DbCommand command, DataSet dataSet, String[] tableNames) at Microsoft.Practices.EnterpriseLibrary.Data.Database.LoadDataSet(DbCommand command, DataSet dataSet, String tableName) at Microsoft.Practices.EnterpriseLibrary.Data.Database.ExecuteDataSet(DbCommand command) at DL.EIN.Exchange.DL.NotesByVisitID(Int32 UserID, Int32 VendorID, Int32 VisitID) in D:\Projects\SVN Code\ENT\36.07\ENT05053607RC\ExchangeDL\Patient\DL.TblPatientMaster.vb:line 4674