If you opened the app and saw that all your videos where gone there's a few reason this might have happened. Depending on how you store your videos this article will outline a few solutions.
Stored in the Photos app
If you deleted the videos from the photos app, or the album they are stored in, they will be deleted from the app. They can usually be recovered from the album "Recently Deleted", if you see the 1 second videos there press restore. If your 1 second videos are in the photos app but doesn't appear inside the app you can add them to your timeline album (by default it's called Leap Second)
Stored in the Leap Second app
If you delete the app and you haven't enabled cloud storage for your timeline the only way to retrieve them is to restore your phone from an backup. You can read more on how to restore from backup