TL;DR: You should be able to resume a zwift activity after a computer crash if you resume it in a few minutes… zwift support says to post this here so it can get votes if people are interested in seeing it happen.
50 minutes into the Road to the Sky, almost up to the Yeti, Zwift froze and my laptop needed to be powered off (hard reset) to recover. When I restarted zwift there was no offer to resume the activity I was in the middle of, nor did it record what I had done so far - all progress was lost. All I got from support was a note about how to prevent this from happening again which had nothing to do with my situation. Things like this are going to happen given, it would be really nice if an activity could be restarted when something goes wrong.