Ok, so there are a few things happening here:
- FC doesn't support cross progression. While you can play on both Xbox and PC, you'll have separate Ultimate Teams on both. You might not have seen your Xbox progress, but it would still be there on your Xbox, if not for the next point.
- Your progress is linked to your EA account. By unlinking your Xbox and linking it to another EA account, any progress you had was left behind. This is mentioned in both our unlinking article as well as the disclaimer you get when you go to unlink an account. You can try to restore that by relinking your Xbox to its original EA account, but there's not a 100% guarantee it'll link together again.
I'd recommend trying to relink your Xbox with its original EA account to try and restore that progress. As for your progress on PC, if you haven't played on it before, you'll need to start with a fresh UT there.