Are trying on the app or the website on a computer? If on the app try the computer instead. If on a computer if using Google Chrome(other browsers may work the same) go in to tool and try clearing the cache, make sure time range says all time. If that doesn't work then try deleting all cookies. If that doesn't work I think I read where someone had suggested in another thread to try using an incognito browser or tab. If you are able to make payment you might want to set it up with autopay and not have to worry about it again or at least until the card expires, then just set up a new form of payment.
If my suggestions don't work then you might need to get a hold of customer service.