Hi there,
Sorry about your problem - it must be really frustrating.
Here's some solutions below that I would use:
- If you go to settings < your name < iCloud < manage storage < backups
Check if you have a backup and you can restore that version. Please note though, that this will replace all current data.
2. go to settings < your name < iCloud < Messages
Here you can see if you have messages syncing with iCloud, toggled on , or off.
A final option, would be to check any other devices that your Apple ID is linked to.
Hope you find your messages!