None examined their own pen? Tell us where you can see how to dismantle it
Post this in the repair forum with pictures of your pen. I don't keep track of modern Italian pens especially Stipula and Delta so I'm not familiar with the model. If the pen is a piston filler (real or captive) and the knob is spinning freely, there is a good chance the spindle has snapped. If the spindle snapped, you will need a replacement part from Stipula if they even have one.