That's a bummer! 

Depending on the size of the card you are using, it is likely that the card was already full and your device has looped over the earlier footage to save the most recent. The looping function works in such a way that the last period of the ride is recorded. You can read more about the looping function here.

Note: If you have the most recent model, check the Manual Footage Capture feature to save and lock video files. This will prevent the device from deleting and looping over important footage.