My fitness data from 2016 to 2019 has completely disappeared after reconnecting with my vivoactive watch on 29 Dec 19

What are you seeing on Garmin Connect web? https://connect.garmin.com 

This typically occurs if you've changed user accounts or if you login with multiple accounts.

On Garmin Connect web I see the exact same data then through the Connect App, i.e. one week instead of four years worth of data. In the Web, I am congratulated for registering my new vivoactive watch (I own this watch since 2016). I have been a Garmin Online user since 2016 first using username and password; approx one year ago I believe I was prompted to change my account credentials to email and password, which I have used ever since without problems. Trouble started after I had to reconnect several times my watch to the Connect App one week ago, probably because of the documented iOS 13 glitches

I have the same problem with my 235, I've lost all of my history and connections. They are gone from the new app and from Garmin Connect

If data is clearly missing from both Garmin Connect Web and Mobile with no explanation, then  this should be reported to Garmin support They should be able to walk through some steps and possibly restore your data from backups.

I have the same issue.  The watch still has record of them but not on my connect account.