Files App

If you open the Excel app, can it see and load the files? If you go to iCloud.com/iCloud Drive are the files there?

Try Settings/Your Name/iCloud and slide off syncing. Wait a few minutes and then slide syncing on.

Last resort - have you tried signing out of iCloud and then sign back in?

If you don't have an app that can use certain files, they will not be shown in the Files app.