imessages

If you had messages in iCloud turned on Keep all your messages in iCloud - Apple Support and those messages were deleted, then they are permanently gone. If that feature wasn't turned on, you can restore from a backup -- Restore your iPhone, iPad, or iPod touch from a backup - Apple Support.

The only way to restore those lost messages would be to erase your phone and set it up with a back-up which contained those messages, if you have one.