I tried several fixes and now it has started recording again but I have lost a weeks worth of data. Will it still be on my Vivoactive3 and how do I upload it as sync isn’t doing it
The activities and health data are synced separately. Vivoactive 3 can store up to 14 days of health data so it should be possible to fetch that data, but there might some issue that causes the data to not sync.
I don't think you can manually upload FIT files with health data to Garmin Connect.