In the time between when the sheet was last opened successfully and when you got this error, did you do anything that would affect Numbers? This would include, and as far as I can imagine, be limited to updating your OS or updating Numbers. If so, it likely did not update properly. Delete Numbers and reinstall.
Do your other existing documents open? If not, this would also suggest a faulty update. But if the problem is limited to this one document, it likely failed to save properly last time you closed it and is now corrupted. If you have backed up your device recently, try restoring from the backup.
Unfortunately this is really all you can do: reinstall the app, and restore a backup. Sometimes data corrupts. I duplicate my most important documents weekly and send from the device to iCloud or vice versa for this reason.