Then I am sorry I can’t help you with that but as I browsed the recent discussions I noticed that you aren’t the only one whose messages got deleted suddenly - so it might be a bug and Apple will hopefully fix it soon. 🙂
Assuming you are talking about the native messages app:
Did you prohibit the app from deleting old messages? (Settings > Messages > Keep Messages > )
This might solve half of your issue.
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Question: messages keep getting deleted?