
Hi,
I understand that some files disappeared in Excel, but just a heads app, Excel app is just used to create a file and it is not responsible on storing the file within the Excel app itself. You can indeed see the list of the Excel file that you just recently created or opened but those files are not saved directly on the Excel, thus, its saved on the location you choose on where you want the file to be saved, either on your Phone or OneDrive.
If you cannot see it on your local storage, try to check on the OneDrive online using the account you used to sign in to Excel app, and see if you can see the files in there .