Immediately check your iCloud or iTunes for a backup that contains 1SE, and restore your device from that backup. We have no way to access or restore your content.
Your 1SE content is stored locally within the app. To be sure your content is kept safe ensure you are keeping a backup that includes 1SE.
Check out this graphic to ensure your content is being backed up (iOS 11):
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.
We know this doesn't help folks who never set up a backup or lost their devices. Please know we're working on a better solution. A solution that happens automatically with our own backup and accounts. The hope is to launch this later this year.
If you're on Android, we have a separate article on restoring.