You need to force the system to take your payment. Try these troubleshooting methods:
Autopay sometimes fails. Maybe when you entered the new credit card, the information did not exactly match your credit card billing address, etc.
Try to manually load funds equivalent to your new plan. See whether that activates your account.
@Jessine This means that if you had a plan change scheduled to go to a higher priced plan, the payment system may have only added enough funds to pay for the older, lower-priced plan. If that happens, you'll need to manually add the difference.