Error: 9004, Severity: 21, State: 1 in sql server

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.

Microsoft suggest 2 options for this 9004 error, let me explain the steps in details. Click here for BOL. 

Option 1: 
First you have to create an empty database with the same name and physical file structure, then shut down the server, swap the both .log and .mdf files you want to attach in place of the empty DB files, and start the server. After above step the database should come up in suspect mode. You can then run this script ALTER DATABASE <database name> SET EMERGENCY to put it in emergency mode, and then run DBCC CHECKDB REPAIR_ALLOW_DATA_LOSS . The above command clear the some data in log to make the database consistent, but may have to delete some data in order to make the database consistent.  This is the best option which is most likely to get the maximum data back. 

Option 2: 
You can attempt to use the CREATE DATABASE FOR ATTACH_REBUILD_LOG to see if that will bring it back.  If the database was cleanly shut down, you MIGHT be able to succeed.  There is also the chance that the database will be inconsistent or corrupt if there are transactions which could not be rolled back.  You should run DBCC CHECKDB REPAIR_ALLOW_DATA_LOSS to make your database consistent. If there were transactions in process no rollback will be possible, so the ALLOW_DATA_LOSS will be required