Hi Peter. Frustrating after all that work. Happened to my friend the other day as well.
He runs his Zwift off of his Iphone to TV. The only thing we could think of that would cause this is a massive internet drop. He took a couple of calls while riding.
You would not notice as your avatar would keep going. All of the surrounding riders would have disappeared temporarily.
You could use https://zwiftalizer.com/ and have a look at your ride file. You can watch this video to learn how to do this. PC or Mac, I believe.