An operation in SQL Server that needs to read or process the transaction log can fail with an error like the following if the transaction log is damaged:
Error: 9004, Severity: 21, State: 1.
An error occurred while processing the log for database 'mydb'. If possible, restore from backup. If a backup is not available, it might be necessary to rebuild the log.
The State number can vary for this error and indicates what type of damage has occurred with the log. See the More Information section about State numbers.
In most cases, this error is just seen in the ERRORLOG or Windows Application Event Log with EventID = 9004 because the operation processing the log is not based on a direct user command (such as recovery running when the SQL Server Engine starts. In these situations this error is often seen with Error 3414). However, some queries such as ALTER DATABASE could require a processing of the log and therefore will see these errors. Since the error is Severity=21, the user session is disconnected.
Error 9004 is a general error indicating the contents of the transaction log are damaged. The reason for the log to become inconsistent are similar to any database corruption problem detected by the SQL Server Engine or DBCC CHECKDB. To find the cause for the damage of the log you should follow the similar techniques for database corruption including an analysis of possible hardware, filesystem, and/or I/O problems. See the Cause section of the following article for more information: How to troubleshoot database consistency errors reported by DBCC CHECKDB.
You should restore from a known good backup to recover from this problem. It is possible that if the transaction log portion of a database backup or the transaction log backup itself has damaged transaction log contents, you can encounter an Error 9004 on RESTORE. In this situation, the transaction log in the backup is damaged.
If you cannot restore from a backup, you may be able to bring the database online by rebuilding the transaction log. You should carefully understand the ramifications of rebuilding the transaction log including the possible loss of transactional consistency in your database. To read about how to rebuild the transaction log, please see the section titled Resolving Database Errors in Emergency Mode in the SQL Server Books Online under the DBCC CHECKDB command
The SQL Server Engine performs logical checks on the consistency of the transaction log contents as it reads and processes it. Not all aspects of the log header, log blocks, and log records are checked. The State number provides more information on what type of failure was encountered when processing the transaction log: