Obviously it’s difficult for me to tell without actually looking at your system personally, but it sounds as though the hdd has either died or developed a fault which is why it would not show up under data storage. All hard discs die eventually.
Ideally, I would want to take the hdd out and connect it to a PC to verify it has a problem.
You have a couple of choices: ask someone more knowledgable to physically check your box and verify the hdd is dead and then fit a new one for you, or ring humax and see if they can do anything for you, or just bite the bullet and buy a new hard disc yourself and fit it. It’s not difficult, but depends how good you are with a screwdriver and electronics.
Usually, when a hdd dies, the data is gone, or it’s just not cost effective to recover.