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.
Click the Subscription tab you'll see their cancelled subscription.
Click the three dot icon to the right.
Then, click Delete.
Then click the three dot icon once again and select Choose Plan to make the subscription active again.
You may need to change the next invoice date to match what it was before you deleted the subscription and then it will be setup like they had never cancelled.