Two points to take into consideration:
- Even if there is a means of using F/R to convert the numbers to superscript there is no way to restore the active fields [relink to the footnote text], &
- How this may have come to happen in the first place.
The only means of restoring actual footnotes is to recreate them.
As for how it happened, my best guess is that the file was opened & saved using some other program that 'claims' to be compatible with Word document formats. That could well negate the styles that control the formatting as well as break the fields responsible
for the active links.
The only other thought that comes to mind is possible document corruption. You might try copying all but the last ¶ then pasting to a new blank document. That may restore the footnotes but I seriously doubt it.