This may not be very easy. I guess you don't have recent backups?

While trying to recover files, you should copy the files to a local drive and ideally work without a network connection. FileMaker 5.5 had the nasty habit of "jumping around" on a network and may open another file with the same name as your FileMaker file. I once had a similar case where the problem turned out to be that FileMaker even ignores the file extension, and thus tried to open a file "filename.pdf" on the desktop rather than "filename.fp5" on the server... IIRC that gave the exact error message you are having. In other words, you should locate and rename any files that have the same name as your FileMaker file.

You could also try to open the files with FileMaker 9, which has a more powerful recovery function (you can even use the free 30-days trial to do this).

If this doesn't work either, contact FileMaker, Inc. - they provide a recovery service for cases like this.