If a user accidentally cancels their subscription and wants to regain access, you can manually reinstate access.

Navigate to the People tab, and under the second People tab, find the user you want reinstate access and click Edit. Under the Access section, click the Subscription tab, and you'll see their canceled subscription. Take not on their next invoice date to change it later. Then click the 3 vertical dots (⋮) and select Delete.

Once it has been deleted, click the 3 vertical dots (⋮) 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 set up as if they had never canceled.