Check Recently Deleted in Messages (open Messages > tap Back/Recently Deleted if available > select the conversation(s) > Recover)
Search for the message thread in Messages and check if it reappears after restarting the app
Restore from an iCloud backup (Settings > [your name] > iCloud > Manage Account Storage > Backups > select device > confirm backup availability; then Settings > General > Transfer or Reset iPhone > Erase All Content and Settings > restore from the relevant iCloud backup during setup)
Restore from a computer backup (connect iPhone to Mac/PC > open Finder (Mac) or iTunes (Windows) > select iPhone > Restore Backup > choose the backup from before deletion)
Check iCloud Messages (Settings > [your name] > iCloud > toggle Messages on)
If the messages were sent via iMessage, check whether the recipient has them (ask the other person to resend or share screenshots)
If you used Messages in iCloud, ensure the same Apple ID is signed in on the iPhone (Settings > [your name])
For specific apps (WhatsApp, Telegram, etc.), check the app’s own “Deleted”/“Trash” or backup/restore options within that app