My old valid card has been declined for unknown reason. I called to my bank, they say CC is OK.
It's best to contact AdWords support directly with an an issue such as this, using the 'Contact Us' link that you'll find on each page of your account.
By the way, even when the bank says a credit card is "OK", there may be other things at work that you won't know about unless you specifically ask. For example a card may be "OK", but the bank may not allow that card to be used for payments made via an automated system such as AdWords, payments from the same source more than x times a month, payments of more than x dollars at one time and so on.
So if you happen to speak to the bank again, be sure to ask if there are any limitations on how (and how often) the card may be used.
AWA
I put three of my clients on invoicing terms, that solved it for them, and for the rest, I made sure there was a backup card to be used in case for some reason the first one wouldn't work.
You can retry the card through the system I think one time I did this several times it fixed the issue after calling the bank. It takes a couple hours to go through if it doesn't do the trick take AA's advice and call a rep.
Called bank, who said there is no problem with the card. Must have been a link failure, they said.
I had a back up card alredy activated so I was safe. Meawhile primary card is again active.
False alarm.