Un-canceling a Stripe subscription

If the subscription is still active, you just have to update the current subscription and pass the plan id once again and it would resume the subscription.

In PHP you would do:

$customer = Stripe_Customer::retrieve("cus_XXX");
$subscription = $customer->subscriptions->retrieve("sub_YYY");
$subscription->plan = "myPlanId";
$subscription->save();

This is covered in one of Stripe's support article in more details here.

Not sure if the accepted answer is an old one, but for the latest version of Stripe you have to do this (in PHP):

$sub->cancel_at_period_end = false;
return $sub->save();