Last updated: 2017-07-19

Source: https://support.freshdesk.com/support/solutions/articles/225260-why-is-the-reply-option-not-available-on-a-few-tickets-created-from-facebook-

If a Facebook page is removed from Freshdesk, all the tickets which were created from that Facebook page will lose connection to that page. For those tickets, the "Reply" button would not appear, so agents will not be able to reply to that ticket anymore.