Did the files disappear after an upgrade?  Simply Fortran does not explicitly manage that list; it is handled only by Windows.  My guess is that, after the upgrade, Windows cleared its own internal list of recent Simply Fortran files.

You should be able to repopulate it by navigating to your project or files in Explorer and double-clicking them.

It's not ideal, but it should work.  I'll look into integrating Simply Fortran's own recent list to the taskbar list.