title says all, in case of hard drive failure, .fit files are lost.
currently there is no way to get them back.
You can export each activity as a TCX file. This should have all of the same details as the FIT file.
Though I think you can only do this one activity at a time, it would be rather time consuming if you have lots of activities.
I would recommend backing up the original FIT files yourself, onto another hard drive, or CD, or flash drive, or cloud storage etc. Preferably several separate backups, in case one doesn't work.