Once you've cancelled your subscription, you will have access to your account and can continue to attend events until the end of your billing cycle. For example, if you're charged for your subscription on April 5th and cancel on April 15th, you can continue to use your subscription through May 4th. During this time you cannot reactivate your subscription - you'll have to wait until the billing cycle is over.
Once you're officially cancelled (after your final cycle), you can reactivate open the app and either hit the Subscribe button in the top right corner or tap to reserve a pass and you'll be prompted to re-subscribe. Please note there is a $49 reactivation fee.