Failure to check-out will result in a fine from the carrier. The costs are deducted from the balance on your card.
Have you forgotten to check out at the end of an NS train journey and are you able to go there the same day? Then try to go back to the station and check out. If you do this within 6 hours after check-in, you will not be fined.
If this is no longer possible, or you have traveled with another carrier, you can request a refund via uitcheckgemist.nl If your refund request is approved, you will receive a message by e-mail from uitcheckgemist.nl that your refund is ready. You then have to go to a OV-chipkaart servicemachine to load the refund on your mobility card.