If a customer accidentally cancels their subscription and wants to regain access, you can manually reinstate access.
Navigate to the customer you want to reinstate access for under People > People and click Edit. Under the access tab you'll see their cancelled subscription. Click the three dot menu and Delete. Then click Choose Plan to make it active again.
You may need to change the next payment date to match what it was before you deleted the subscription and then it will be setup like they had never cancelled.