Immediately check your iCloud or iTunes for a backup that contains 1SE. This backup is what you'd need to restore from. Go to Settings > [your name] > iCloud > iCloud Backup to find a backup with 1SE.
VERY IMPORTANT: Please keep in mind logging into iCloud isn't the same as an iCloud backup. 1SE is NOT saved in your iCloud account. Only basic data like notes, mail, and contacts are. You'll need to do a backup via iCloud or iTunes.
Not sure how to find this?
Here's a graphic for finding an iCloud backup with 1SE (iOS 11):
Found a backup but not sure how to restore?
We know this can involve a lot of steps and have created step by step guides and videos to help with restoring.
While this doesn't help folks who don't have a backup or lost their devices, please know we're working on a better solution. One that happens automatically with our own backup and accounts.
If you're on Android, we have a separate article on restoring.