Changed file extension-can't fix

I opened a .XLSX file in Numbers and did some work in the spreadsheet. I then saved it to the cloud. Later I tried to email it, but was told it had a 0 byte size and could not attach it. I didn't see a file extension so I manually added a .XLS.

The Mac gave me some alert, which I don't recall the content, but I went ahead and made the change. The file then became a folder, and I can't open or restore it.

Any ideas???

Thanks,