If you bought your tickets from or through a third party and you were denied access to the venue or the event, and the box office staff at the venue could not help you either, then you should claim your money back from the seller.
Perhaps you were offered a false or falsified ticket, a ticket that was resold several times or a ticket that is already part of an official investigation into the seller.
You can file a complaint against this seller via the button on www.ilovemyticket.com.
NOTE: Many resellers work from abroad (even if they call themselves for instance ticketsBelgium) and therefore slip through the net of the Belgian legislator.
If you paid for your tickets with a credit card, you can also lodge a complaint through that channel (bank, ATOS, Amex...) and demand your money back.
If you confront the seller himself, he will often demand that you provide proof that you did not get in.
The Tele Ticket Service staff at the box office can confirm that you were refused entry. This may help you when you want to claim your money back from the retailer or when you make a complaint through another channel.
Websites where you can complain against rogue ticket sellers: