No data between two dates.

Data shouldn't just disappear. Consider that you might have been logged in using a different Garmin account during that time window. Data is Missing from My Garmin Connect Account After Syncing | Garmin Customer Support

You can search for some of your known current and past email addresses using the Connections Search on Garmin Connect Web.

https://connect.garmin.com/modern/connections/search

1690367 said:

how can I connect by chance with another account?

It can happen for example, when reinstalling the app or getting a new phone. It's easy to make the mistake of creating a new account rather than using your existing account.

Suggest contacting Garmin Support via phone or chat for additional assistance.

Never installed or re-installed  or change account twice or got  new phone in-between. Alzeimer syndrom?

Check that : I did nothing between 11th ant 12th about phone or Garmin account , absolutely nothing playing with the garmin tools.Only wearing the watch on my wrist like every day of my life the past two years.

I think that a part of the data didn't recover when I installed the App a few days ago on my phone For me it was a question of time that they load little by little, but 5 days later, it becomes unlikely. 

Which watch do you have?

Have you been syncing with the tablet continously during that time? The watches do not have unlimited memory for activities and health data. If you don't sync often enough (how often depends on the watch model) that data will be overwritten and lost.

Hello, my watch is a Vivosport, this explanation suits better. Yes the tablet has been "off" for several weeks, I don't remember how many. And I haven't synchronised the data until I felt the need to check my perf of the year, and was forced to get the App on my phone. I'm sad  to lose it, I thought maybe the data was stored somehere in a cloud during a while and can be sync Serve me as a lesson then.

C A

Vivosport will store: 7 timed activities, 14 days of activity tracking data

You need to sync at least every 14 days to not lose any health data like HR, steps etc. The activity memory will only store 7 activities so how often you need to sync that depends on how often you log activities.