Check the Messages app on your iPhone, iPad, or Mac
Search for the contact name, phone number, or keywords in Messages
Turn on Messages in iCloud on all your Apple devices
Sign in with the same Apple ID on all devices
Restore your iPhone from an iCloud backup that contains the old iMessages
Restore your iPhone from a computer backup made in Finder or iTunes
Check archived or synced messages on your Mac
Use Spotlight search on Mac to find old message content
Check if the conversation was deleted only on one device
Ask the other person to resend the message thread or screenshots
Contact Apple Support if messages should be present but are missing