When you complete a ride, it automatically uploads to the Zwift Dashboard if you choose "Save" versus "Discard." Unfortunately, if you experience an internet dropout while saving, it can result in a partially uploaded Zwift ride (as opposed to the ride itself being completely lost). Rides are uploaded to the server incrementally every 10 min.
If this affects the automatic posting of rides to 3rd party services you have connected, like Strava, you can find a copy of your complete FIT files locally on your device as described in this article.
There isn't a way to manually upload the full ride to complete it in Zwift (yet), but we're exploring adding this feature in the future.
How to Prevent Partial Rides
If you're disconnected from the Internet during a ride, you'll see your rider list empty out, and it will look like you're riding alone. If this happens, try to get your Internet to reconnect before you end your ride.
If you're on WiFi, you can also try to strengthen your WiFi signal - either by moving your router or your computer closer to your router - or using a range extender. Alternately, you may want to run an Ethernet cable to your computer instead.