The reason for this is most probably an internal wire that snapped (or the joint of the wires/internal mechanism that was holding it in its place), and that is not a thing you can repair by yourselves, I'm afraid. The wire or joint/púastic component holding it together will need to be replaced, most probably. So you can only try the customer service some more or try to find a stroller repair service or a Maxi-Cosi distributor around you. I am sorry for bringing up bad news but even I wouldn't know how to repair this by myself, without having the proper training and/or parts.
Btw, it is not your fault, I believe you were gentle with it. These models are made to be on the cheaper side, and having about two years, the component probably just wore down. I would also not use it anymore while the thing is not repaired as it can be a health hazard for your little one, the folding while still inside.