Are you using Backup and Restore (Windows 7) in Control Panel, and saving to an external drive or a network location?
Please check that you can open the backup destination in File Explorer and copy a small file to it. If it’s a USB drive, try connecting it directly to another USB port
Could you share the full error message or a screenshot? That will help confirm whether Windows is losing access to the backup destination
So, consider also to use another backup software, this feature (backup and restore windows 7) isn't actively developed by Microsoft
Let me know
Elvis