I Think that there might be problem in the file permissions for that Alert file because of which it might not be able to update the file.
It is Also possible that the file might have been moved or corrupted.
If this is the case a new alert log file might be created in the same destination which will alow the LGWR to update the log file whenever it tries to update the file.
Please let me know if it helps in solving th problem
I really don't know what might be the problem, since the DB has been started and working fine for more than a month, so there is not problem with the permissions.
My teory i that maybe Oracle tries to use this file to write log records, but it might be still used by some other Oracle process so the OS (WinNT) does not allow it to be accessed. There was a quite big transaction going on while this happened. (But transactions like that one happened almost every day on this system for a month and it was fine till this happened).
To solve the problem I dropped that log file (ALTER DATABASE DROP LOGFILE...) and created 8 new logfiles, so thus might prevent overlapping the processes that requuest same log file.
Maybe is some bug of oracle...maybe it is a Winodws NT bug...
******* EXCERPT FROM THE ALERT LOG FILE *****************
Errors in file D:\Oracle\admin\PROD\bdump\prodLGWR.TRC:
ORA-00321: log 19 of thread 1, cannot update log file header
ORA-00312: online log 19 thread 1: 'D:\ORACLE\ORADATA\LOG19.ORA'
ORA-27091: skgfqio: unable to queue I/O
OSD-04008: WriteFile() failure, unable to write to file
O/S-Error: (OS 5) Access is denied.