Make sure you request your refund within three months. It works as follows:
Step 1: register
Go to uitcheckgemist.nl and register using your OV-chipkaart number If you have a personal OV-chipkaart, enter your birth date.
Step 2: enter info
The website lists all rides without a check out. Note: It may take up to 14 days before you see the missed check out displayed. If your trip is not shown, try again a few days later.
Do you want to request a refund for a trip on a balance? In so, please enter the boarding location and final destination. The balance is immediately visible.
Step 3: collect
Your requested refund will be prepared for your card. You put it back on your OV-chipkaart at a Ticket Vending Machine. Afterwards, you can use the balance to travel.