Originally Posted by JanG

Have you tried doing a hard reset? Disconnect the battery, leave it for a while, then reconnect the battery. Do not switch on the device, but plug it into a wall charger and let it recharge until full, then try and start it up again.

I'm not saying this will fix the problem, but it's something you may not have yet tried.

Also, when you were deleting books off the SD card, did you by any chance unplug the Gen 3 from the USB without doing a safe removal?

--Jan.