It sounds to me as though the document has become corrupt. You can try and download it from one device or another, however this may be unsuccessful.
Whilst it won't help you recover documents that have already become corrupt, can I suggest you get into the habit of backing up your documents in the cloud regularly.
Thanks for the reminder! Until reading your post I didn't think about my backups.
There was only one critical spreadsheet that could not be easily recreated. I had a recent backup in Dropbox. It is now uploaded to iCloud and I'm back to work.
I have not yet read your iCloud backup tutorial in its entirety. I'll get to that over the weekend. Thanks again.