If the credit was on the card when you activated NS Flex, it should be refunded to you on the invoice after the first month.

If you loaded the credit on the card after activating NS Flex, you can go to a service desk and have it paid out to you.

What if I am already not there, can I request the money online?

Hi JavierMarcos!

It is possible to get the request a refund online for the balance of your OV-card. But the only way to do that, is to block the OV-card. If it is possible for you, my is advice is to go to a service desk on the station. 

I think JavierMarcos is not in the Netherlands anymore.

Hello 

please I want refund back to my account 

I already cancel my monthly plans 

please can you help me 

Hi S.I.ichaba and welcome to the NS Community! 
Do you mean a refund for the contract costs? In that case, you can contact our costumer service to ask about that.