@meg4 wait an hour and try to change the card again
if still fails, open ticket with support for help:
@meg4 you have the $55 plan with $20 credit?
if a payment was missed due to Autopay failure or other reason , it is NORMAL that you have to first pay the full amount of the plan ($55) to resume the serivce first. But no worry, the $20 will be there. After couple days, PM will credit you back the $20 into your account as Available Fund. So, you didn't loss the promotion. Just that you have to pay the full amount first and get the credit after