Starting November 28th, 2022, free databases in personal accounts are queued for deletion. Contact Heroku Support as soon as possible to recover your data before it is actually deleted.
After data has been removed from Heroku, it can't be restored. It's vitally important that you upgrade your database to a paid plan prior to November 28, 2022 and that you back up your database prior to the upgrade.
See our blog post and the Removal of Heroku Free Product Plans FAQ for more info related to the ending of free plans.