The same what happening to me. The app never seemed to finish downloading in the Purchased tab. But when I checked it on the Home Screen there it was, with a blue dot. I tapped on it, and it opened as normal. Every thing looks proper now.
If this does not work for you, you may have to delete and re-download the app. If you are using iCloud Drive to sync/save the spreadsheets they will re-sync when you turn iCloud Drive back on in Numbers after re-downloading the app.
If you are not using iCloud Drive, then you should export/save those files (as Numbers files) to some other location; DropBox, your computer, etc. It will be a pain exporting and re-importing them, but better than losing them.