The belt must be replaced
First of all, open the box, rewind the shutter and lock it so that it cannot unroll (a screwdriver between the slats and the box for example). Now remove the belt from the pulley, open the winder by unscrewing the plate screws and remove the belt. Replace it by checking that the new one is the same length as the old one. Reassemble the winder by pretensioning the spring in order to recover the belt and connect it to the pulley on the roller.
The strap does not rewind
The reel spring has probably decided to abandon us, you can try increasing the tension by making a few more turns before reconnecting the belt or replacing the reel directly.
I have replaced the belt but a lowered shutter advances a piece
The belt is too long, consider how much and cut off the excess.
There is a broken cue
You will have to let the roller shutter unfold and disconnect the binding straps, at this point it is possible to remove the roller shutter from above to replace the damaged slats.