Did you load an NS Flex season ticket on the card? With NS Flex you don’t need any balance anymore, since you will get an invoice at the end of each month for all the costs incurred, and these will be automatically withdrawn from your bank account. The balance that was still on the card when you loaded NS Flex onto it will be subtracted from the first invoice, i.e. after the first month of travel.
If you didn’t load an NS Flex season ticket, look at the travel history on www.ov-chipkaart.nl. There you can see all transactions and find out what happened to your balance.