Check whether the messages are still on another Apple device signed in with the same Apple ID
Turn on Messages in iCloud on all your Apple devices
Restore your iPhone from an iCloud backup made before the messages were deleted
Restore your iPhone from a computer backup made before the messages were deleted
Sign in to the same Apple ID and verify the correct phone number and email addresses are enabled for iMessage
Check the Recently Deleted folder in the Messages app if your iOS version supports it
Ask the other person in the conversation to resend the messages or screenshots
Contact Apple Support if the messages were tied to an account or sync issue