When a Stripe subscription is canceled for the end of the period it can be resumed before the cancellation is complete.

I’m using Nova Framework not setup with Nova? please read Getting Stripe API setup with Nova Framework

A subscription that has passed its cancel window or has been canceled immediately cannot be resumed. To resume a subscription the process is to retrieve the subscription by its id then assign the plan id of the original plan and save.  

First get the customers information, if you don’t have this method please read Stripe API get customer information

From the $customer the subscription can be retrieved. In this case, the plan_id comes from a database, adding the same plan id resumes the subscription.

From here it's a good idea to update any information you are storing such as the end dates and status.

$customer = $this->getCustomer();

if ($customer != null) {

    if (isset($customer->subscriptions->data[0])) {
        $subscription_id = $customer->subscriptions->data[0]->id;

        //resume
        $stripe = $customer->subscriptions->retrieve($subscription_id);
        $stripe->plan = Auth::user()->plan_id;
        $result = $stripe->save();

        $user = User::find(Auth::id());
        $user->current_period_end = date('Y-m-d H:i:s', $result->current_period_end);
        if ($result->canceled_at) {
            $user->canceled_at = date('Y-m-d H:i:s', $result->canceled_at);
        }
        $user->cancel_at_period_end = ($result->cancel_at_period_end) ? 'true' : 'false';

        $user->status = $result->status;
        $user->save();

    }
}