Note: Day One Classic (version 1.x) has not been updated since March 2016. It has been retired. We strongly recommend updating to the current version of Day One. See here.
Some Day One users who sync with iCloud have reported that their journal is missing:
Go to iOS Settings > Account (Your name at the top of the Settings screen) > iCloud on your affected iOS devices.
Be sure iCloud Drive is enabled in this list
Find Day One in the list of apps, then toggle it on.
Open the Day One Classic app and go to Settings > Sync, then enable iCloud. (The screen shot below shows this setting as disabled, but it needs to be enabled).
Confirm that your content has reappeared in your Timeline.