I'm not sure that Message history is backed up; but that is where it would be if anywhere.
Check the dates of your last backups. If the date is after the conversation, but before you deleted it, you might be in luck. Be aware that a restore from backup is all or nothing. It will remove all app data from after that date.
Depending on where you've got your backups set to save:
On the computer, check Preferences, the Devices tab.
On the iPad, check Settings > iCloud > Storage & Backup > Manage Storage; then your device in the Backups list.
Note if you've got iCloud backups going, then it usually does an auto-backup at night when you've got it charging, so it is probably already too late.