When an application for any visa is rejected, including your Belgium spouse visa, the first thing you should do is return to the body with which you made the original application.
It's difficult for anyone to know exactly why your application case ended in this way, but the important thing to remember is there is still plenty of opportunity to appeal, and that a majority of visa applications end in rejection at the first time of trying.. many more go on to appeal, and are successful!
In the meantime, here's a list of some of the documents applicants must provide to obtain a Belgian spouse visa:
• A passport of other travel documents.
• Two visa application forms that have been filled out and signed by you, accompanied by three recent colour photographs of you.
• A copy of your marriage notice form. This must have been issued in the last six months by a Belgian registrar.
• Proof of your financial means in the form of payslips, bank statements or income tax papers.
• A copy of the Belgian identity card carried by the spouse residing in Belgium.
• Proof that the relationship is genuine and longterm. E.g. Letters and plane tickets.