Wow, Alessandro. That reed got pushed way down in there. I’ve had this happen a few times, to a lesser degree and have managed to work starting near the base of the reed with a needle and then an injector razorblade to finally pry the end back up.
My guess from what you say is that the reed is shot. A single reed can be replaced, but probably neither you nor I would be able to do that. Your best bet, in my opinion, is to acquire another Hohner HM-26, the regular one that corresponds to the Cassotto 26. That will be easier to find and cheaper than a Cassotto. It is exactly the same as the Cassotto, except for the silver box (“cassotto”) on the bottom. Otherwise, the parts are interchangeable. What you’ll do is to mount the box on the HM-26 and it will be “cassotto-ized.” I have actually done this with a Cassotto 26 because of reed issues. It plays and sounds no different from an original Cassotto except that your new one doesn’t have the logo.
The alternative is to remove the regular HM-26’s reed plate and use it to replace the damaged reed plate of the Cassotto-26. This is doable. I have replaced reed plates, but I wasn’t confident about taking that approach with my Cassotto.
Finally, it’s possible that you could order new reed plates from Hohner. I’m guessing this might be more expensive than harvesting them from a used instrument.
There are HM-26 Hohners out there, as you probably know. You might have to watch eBay for a bit to find one at a reasonable price. Some of them are old-stock new. I have an extra HM-26 or two, though I’m not looking to sell them, and shipping to Italy would be prohibitive.
Maybe someone will be able to suggest a better approach. If a reed jumps from A4 to G-5, it is a goner.You’d need a chain saw to tune that back down to A4.