Thanks everyone for your help!
So far nothing have changed. After charging around 12h I've tried the reset propose by Semwize and also holding 20s (as suggested by davidfor). Nothing changed. (FYI, holding the button 5s the led goes off and holding it 2 seconds light up blinking once)
I've burnt the new image (thanks davidfor) into the microSD (using dd command, i'm on a linux machine). Still stuck in the same initial black restoring screen. The reader does not seem to be either booting... or at least is not communicating with the screen
Running out of ideas, I'm thinking of unplugging the battery for a while to check if something changes
Any other advice? Thanks in advance!