Fixing corrupted Firebird database
Here's a short step-by-step walkthrough:


disconnect users and disable incoming connections to the database 
make a copy of database file (or two copies) and work on that
use GFIX with -v option to validate the database file
use GFIX with -v and -f to do full validation


If problem is not too serious, you can try to backup the broken db and restore under a new name:


use GFIX -mend to prepare corrupt database for backup
use GBAK -b -g to backup the database. -g disables garbage collection 
use GBAK -c to restore backup to a new database.


If you succeed, you have fixed the problem and have a functional database. If not, you can try to create an empty database with the same structure and pump the data to it .

One of the reasons why backup or restore can fail is if some broken database triggers exist, and prevent connection to the database. For example, a database trigger might use some table which has a broken index, etc. To work around this, connect to database with isql tool using -nodbtriggers option and then disable those triggers. You can enable them later when you fix other problems and get a working database again.

Another reason restore might fail is when you have broken data, so some of validity constraints (check constraints, etc.) cannot be satisfied. In this case, you can try to restore your database using -N[O_VALIDITY] command switch to gbak.

If you're interested in a more detailed information of the process of fixing the database, as well as explanation of some types of corruption, take a look at the following page:

http://www.ibphoenix.com/resources/documents/search/doc_5

If all fails, you can try IBSurgeon tool, which is able to fix most problems and extract data. Also, IBSurgeon's website has a detailed
explanation of causes of database corruption and ways to fix it:

http://ib-aid.com/option,com_content/task,view/id,58/Itemid,62/ 

check this guide http://www.firebirdfaq.org/faq324/