I just logged in tonight for the first time in a couple months and it welcomed me as if I was a new member and showed no activities. It seems to be syncing stuff now, but I’m wondering what happened? It shows that I have runs in 1969 - before I was born! 
Is this some kind of Unix bug?
From my side, it looked like you de-authorized CityStrides from having access to your MapMyRun account. When that happens, all your data is removed.
Did you go into the ‘Manage Applications’ section of MapMyRun/MapMyFitness settings and revoke CityStrides? That would have been done here: MapMyRun
How’s your data looking now? Do you still have runs from 1969 (if you do, can you share a link to one)?