To fully understand what this means, you need to know that the shop and the
administration view of the items do not access the individual item, but only a
summary generated from the item data. So if you have manipulated your data with FTP,
the list will no longer tie in with the actual item list. You will have to recreate
this list or index.
If some or even all of the items in your shop seem to have disappeared suddenly and
you are sure you haven't accidentally deleted them, then the list either wasn't
generated or hadn't been generated properly. You can bring about the recreation of
the list by clicking 'Update item index'.