If your Pro subscription has already expired, you can always renew it in the Manychat settings.
Please, open the Settings tab, go to Billing, and click "Renew subscription". Your page will be back to Pro right away! Here you can see how it looks:
Also, if there have been a few attempts of charging, but they haven't gone through, instead of "Renew subscription" there would be another button - "Update Billing Credentials". In this case, you'll have to manually enter your card's billing data, including its number and CVC-code, Street, Postcode, City, and the Country.
Hope this helps!