Retention tags in Exchange do not help with data recovery, they are used to delete/move data. Still, the deleted contacts should be recoverable for up to 14 days after the incident, 30 if you have increased the Single-Item recovery window to the maximum value. If you are not able to find them in the Deleted Items/Recover deleted items container, then eDiscovery is your only option. You can search for "kind:contacts" to include all contacts.
Again, this will only work for a limited time if the contacts have been deleted, unless you have put the mailbox on hold.
