Can I reactive a subscription that has been cancelled with Authorize.net?

Per Authorize.net, it is not possible to reactivate a terminated subscription. You can see further details here:

https://account.authorize.net/help/Tools/Automated_Recurring_Billing/ARB_Subscription_Detail/Reactivate_a_Suspended_Subscription.htm

You can, however, create a new subscription, or ARB, from a previously successful transaction. Steps are outlined here:

https://support.authorize.net/knowledgebase/Knowledgearticle/?code=KA-04444

Once that is done, you can create a new offline Transaction record in Soapbox to prepare it to receive notifications of subsequent recurring payments:

Creating, editing, and filtering offline transactions in the Transactions view