You can find all of your past activities in Zwift on your Dashboard at https://my.zwift.com/ and in the Activities page of the Zwift Companion app.
At the top of the Dashboard, you'll see cumulative stats (levels, distance, days, pizza slices) for your cycling activity.
Note: While your running activities will show up in the list, your running stats (levels, distance, days, pizza slices) are not currently visible due to a known issue.
The Activities page in Zwift Companion can be accessed by simply tapping your profile picture and then tapping the orange Activities button on your profile.
If you need to manually upload Zwift rides to third-party sites, you can download the .fit files from here! Steps for manually uploading will vary from site-to-site; please consult the individual service's instructions.
Example .fit file download link location on my.zwift.com
Note: If Zwift crashed or was disconnected from the Internet before it could upload your .fit file, the file you download may be incomplete or malformed. Learn more about partial and malformed .fit files here.