This particular site has been running for 2-3 years and yesterday started showing this error:  Any help would be appreciated.

Server Error in '/' Application.

Cannot open user default database. Login failed.
Login failed for user 'NT AUTHORITY\NETWORK SERVICE'.

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: Cannot open user default database. Login failed.
Login failed for user 'NT AUTHORITY\NETWORK SERVICE'.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.  

Stack Trace: 


[SqlException (0x80131904): Cannot open user default database. Login failed.
Login failed for user 'NT AUTHORITY\NETWORK SERVICE'.]
   System.Data.ProviderBase.DbConnectionPool.GetConnection(DbConnection owningObject) +428
   System.Data.ProviderBase.DbConnectionFactory.GetConnection(DbConnection owningConnection) +65
   System.Data.ProviderBase.DbConnectionClosed.OpenConnection(DbConnection outerConnection, DbConnectionFactory connectionFactory) +117
   System.Data.SqlClient.SqlConnection.Open() +122
   Microsoft.ApplicationBlocks.Data.SqlHelperParameterCache.DiscoverSpParameterSet(SqlConnection connection, String spName, Boolean includeReturnValueParameter, Object[] parameterValues) +233
   Microsoft.ApplicationBlocks.Data.SqlHelperParameterCache.GetSpParameterSetInternal(SqlConnection connection, String spName, Boolean includeReturnValueParameter) +343
   Microsoft.ApplicationBlocks.Data.SqlHelperParameterCache.GetSpParameterSet(String connectionString, String spName, Boolean includeReturnValueParameter) +175
   Microsoft.ApplicationBlocks.Data.SqlHelperParameterCache.GetSpParameterSet(String connectionString, String spName) +45
   Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteReader(String connectionString, String spName, Object[] parameterValues) +207
   DotNetNuke.Services.Log.EventLog.DBLoggingProvider.Data.SqlDataProvider.GetLogTypeConfigInfo() +71
   DotNetNuke.Services.Log.EventLog.DBLoggingProvider.DBLoggingProvider.GetLogTypeConfigInfo() +101
   DotNetNuke.Services.Log.EventLog.DBLoggingProvider.DBLoggingProvider.GetLogTypeConfigInfoByKey(String LogTypeKey, String LogTypePortalID) +65
   DotNetNuke.Services.Log.EventLog.DBLoggingProvider.DBLoggingProvider.AddLog(LogInfo objLogInfo) +75
   DotNetNuke.Services.Log.EventLog.LogController.AddLog(LogInfo objLogInfo) +193

 


--------------------------------------------------------------------------------
Version Information: Microsoft .NET Framework Version:2.0.50727.3082; ASP.NET Version:2.0.50727.3082