Q: My credit card ran out of money

If you have a credit card on your account then it has to be valid. If you don't have another card that you could replace it with try removing it from your account, that will allow yo to download free items.

On your computer's iTunes you should be able to edit your payment info by going into the Store > View Account menu option and logging into your account, and on your account's details page there should be a payment link.  If you are doing it on your phone then tap on your id in Settings > iTunes & App Store and tap on 'View Apple ID' on the popup and log into your account  - that should also give you a payments link on your account's page.

Changing payment info : Change or remove your payment information from your iTunes Store account (Apple ID) - Apple Support

If you don't get the 'none' option to be able to remove the card : Why can’t I select None when I edit my Apple ID payment information? - Apple Support