You export data by opening your form, then go to Tools>Forms>More Forms Options>Export Data.
You will have the choice of four different formats: FDF, XFDF, XML and text files. You can also select to merge one or more files into a spreadsheet (actually, a CSV file, but that will open in e.g. Excel).
To import data, you would use Tools>Forms>More Forms Options>Import Data (again, with different options).
So, try to export the data as e.g. text and see if you can actually read the contents of the file. If that does not work, then the data in your file is corrupt, and you don't have much hope to recover it.