If you (or a member) accidentally cancelled a customer's subscription then providing the subscription in choice has not reached its original renewal date, it can be 'uncancelled'.

Note: When a Subscription cancellation request is made, it is marked as 'pending cancellation'. If you change your mind, the subscription may be restored before its scheduled cancellation date - e.g. the date on which it would renew.

Navigate to customers profile > Memberships and find the subscription in question, click on the 3 dots … to the right of the price information and click 'Keep subscription'. The subscription will then renew at the original renewal date.