Of course the obvious answer is restore from back up.
The next answer is that assuming you are getting a specific error when you try to open the database.
There use to be time when the database would be corrupted and you would get an error stating that the file was not a database. If this is happening, you would go in with a hex editor and change a value so that FP would recognize the file as a database and then go in an retreive your data.
The exact values and information have long ago slipped from my memory. You should be able to check the Internet for the steps.