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
Regards,
Vishywish