A component like that should not come loose. Barrel, section and metal fittings are permanently joined at the factory, and if the joint opens during normal unscrewing and screwing up, that counts as a manufacturing fault and not as mishandling — even if the barrel had previously been screwed shut rather firmly. Whether the parts are glued or press-fitted cannot be settled from the outside.
The important thing is to leave the pen exactly as you found it and on no account to glue it yourself: traces of adhesive make a proper repair considerably harder and can forfeit any goodwill. Any ink left in the pen should be removed beforehand.
The route to take is through the authorised dealer or the Montblanc boutique where the pen was bought; from there it will be sent in for repair. In comparable cases such repairs have been carried out free of charge, sometimes because the manufacturer acknowledged the fault, sometimes because the dealer covered the cost. Outside the guarantee period there is no entitlement to this, but it is always worth asking, and in any case only the workshop can give a binding figure once it has examined the pen.