Originally Posted by macropod

Was the file on a USB drive? If so, the most likely explanation is that the drive was removed without going through the 'safe removal/eject' process. That regularly accounts for lost & corrupted Word files, though I don't know why it would affect backup copies - unless a fresh backup had been made and not closed when the drive was removed. Time to restore your file from your last known good backup or an email copy.

Other than that, your Mac might have an OS error and/or a faulty Office installation. Are both fully-updated? A reinstall of Office would be required to fix it if it's faulty.