Check the Messages app and scroll to the conversation
Use the search bar in Messages to find keywords, names, or dates
Check if messages are on another Apple device signed in with the same Apple ID
Restore from an iCloud backup that was made before the messages were deleted
Restore from a computer backup made with Finder on Mac or iTunes on Windows
Turn on Messages in iCloud if it was previously enabled and sync the device
Check the Recently Deleted folder in Messages if your iPhone version supports it
Ask your carrier if they provide SMS message records
Use a third-party iPhone data recovery tool if you have no backup
Contact Apple Support if the messages are still missing