Q: My icloud randomly stopped backing up 3 weeks ago.  Wont do automatically or manually.

Did you check if there is enough free space available?

Manage your iCloud storage - Apple Support

Can you back up to iTunes on your computer?

If yes, create a backup in iTunes, then delete the iCloud backup in Settings/iCloud/ and try again to start a manual backup. If you are still getting the error message, you'll have to contact Apple Support and talk to the iCloud specialists. They can look into your account and storage info from their side.

If your iCloud backup size is 4GB, did you try to backup just the iOS data, without photos, or other 3rd party app data, in case some of this data is damaged?