Door Security

Is there any indication on the existing door who the manufacturer is? If so, contact them first.

There are a huge variety of locks and handles available at places like Brico, to change the cylinder on just about any lock these days, even high security locks, is a 15 minute job involving one screw. If you really need to change the complete lock (as opposed to switching out the cylinder) then that is perhaps another 10 mins and 2-3 screws.

Otherwise, if you can't do this yourself, contact a locksmith.

http://www.stevens-locks.com/
http://www.serrureriemertens.be/
https://www.serrurerie-andre.com/
http://www.alliance-artisans.be/serrurier/ixelles
http://www.serruriersprl.be/serrurier-ixelles/

Just be sure if you contact any of these that you tell them it is not an emergency. Otherwise, they'll turn up fast and charge you hundreds of Euro's per hour for their work.

Telling us where you live will help - though Anon seems to think it is Brussels.
Most important, is it your property or rented?
If rented you need the owner's permission and you MUST give them a key or they are legally entitled in an emergency to break the door down and charge you for the damage.