Update: We've found that this issue is due to a combination of factors related to losing your Internet connection. When Zwift tries to reconnect, it can freeze the game. If you encounter this, please send us your log.txt (located in your Documents\Zwift\Logs folder) from the ride where your game froze so we can confirm.
We've found that some combination of game and network issues occasionally cause Zwift to freeze. This may happen immediately after pairing or during a ride. If the game freezes, it is likely to do so for an extended period of time; we've observed this behavior continuing for up to 10 minutes.
It's possible Zwift may recover from the freeze and resume, but we've also observed it continuing indefinitely.
Our development team is actively investigating this issue, and we'll update this post when we have more information. Thanks for your patience and understanding as we get this sorted!
Your best preventative measure to avoiding this issue is to strengthen your Internet connection so that Zwift is less likely to need to reconnect. If possible, run an Ethernet cable directly from your router to your computer. If you need to connect via WiFi, move your computer closer to your router - or consider using WiFi range extenders to improve your signal coverage.