I've also had this problem with activities not populating the activity graph/total activity duration report/activity calories report/activity distance report.
Thursday was the last day that activities were charted correctly (although they are all showing correctly on the Activities list)
It can only be a simple database query that Garmin has modified to render the whole lot as useless.
I'm hoping that they were fixing the previous issues with reports that only certain activities would show up and broke everything instead. At least it means they're finally working on it :D