If the courier hasn't picked up your parcel yet, contact the Box Mayor and ask him to open the Box. Don't worry, your reservation will still be valid.
If the courier has already picked up your parcel, get in touch with them in order to find out where your parcel is. At this time, we're working with bpost in Belgium. Find their contact details here: http://www.bpost.be/nl/faqs#contact-section-faq.