Not knowing that much about it, I would guess it might a lose screen cable inside. But others know a whole more about this than I do.
If you're in love the Kobo Glo HD (and this one can't be fixed), Rakuten (themselves) sell refurbished Glo HDs on eBay.
https://www.ebay.com/itm/Kobo-Glo-HD...AAAOSwKXdahfL7
EDIT: Just realized you were in Spain. I know Rakuten sells (or used to sell) these refurbished readers on some European eBay sites, but I don't see Spain included.