Hi kgxxlu and welcome to the NS Community! 

It depends on what kind of ticket you have and what you did whit it. If you bought a ticket from the machine on the station and you didn't check in, we can check that in our system. If you didn't check in with this ticket, you can contact our customer service for a refund. 

If you bought an e-ticket (online), you can only request a refund when you didn't open the ticket. You can also contact our customer service for this. If you want to buy a ticket, but you are not sure if you're going to travel, then my advice it to purchase the ticket on the day you want to travel.

It's already stated here many times: don't buy etickets in advance because they can't be changed or cancelled