[DJUNGELSKOG]

My very old Pocketbook had these exact symptoms at some point. It wouldn't turn on and nothing happened when I connected it to a laptop or to a wall charger. I put it in a drawer.

Two years later I wanted to recycle it so I tried turning it on just to double-check that it's truly dead. I connected it to a laptop... and the device actually turned on but its battery was in a very poor shape. It looks like something went wrong with the battery and letting it fully discharge has "fixed" it. Luckily I was able to buy a replacement battery at Subtel, as a bonus, the device's clock now works again after being broken for years.

TL;DR: try replacing your battery.