If a customer's membership shows 'Registrations Blocked', it means that the membership is unable to be used to make bookings due to the failed payment(s) on the membership.
In order to remove the block and allow your customer to begin using the membership again, you will need to resolve all failed payments on the membership.
To do this, go to the customer's profile >> Memberships tab >> click on the membership >> Resolve next to the failed payment.
If the payment failed using an online payment method, you can choose to 'Void' if you don't want to take it, 'Confirm' if it has been paid, or 'Change Payment Method' if they are going to pay using an offline payment method.
If the payment failed due to no payment method added, you will have the option to 'Change Payment Method' or 'Skip/Edit Payment'.
Repeat this for each failed payment on the membership. Only clearing the most recent payment won't unblock the membership, all payments need to be resolved.