When the batteries run low on my JBL, I change them out for the extra set I keep on hand. I insert the batteries (rechargeables) properly and replace the cover, then press the "on" button. And nothing happens. the first time this really alarmed me. I even tried the extra camera batteries (AAs) I had with me (I was on the bus), but they didn't work either -- three sets of dead batteries?!?! Argh!
But I know by now that the batteries aren't the problem, and the only way to wake the JBL back up is to hook it to my computer (at home or at work, doesn't matter, and I can use a spare mini usb cable, too). The computer recognizes the device (or at work it doesn't quite, but that doesn't matter), and the JBL comes on. It has "forgotten" what book I'm reading and where I was in that book, and it may not be able to see some of the books on my card... but at least it comes on and works properly -- especially after I check the card using Windows Explorer to see the books are there, then the JBL sees them, too... until the next time the batteries are too low.
I'll have to train myself to change out batteries when first warned they're getting low and when I'm at home. I'd hate to be lugging the thing on a long bus trip to the other side of town, only to find that I can't turn it on again even with fresh batteries. It almost makes me think of getting a different reader... but that would need juicing up from time to time as well, thus having the same basic problem.
Note that I've run out of battery power for my camera in the middle of Balloon Fiesta, but changing batteries in that makes it all better again. Maybe if Canon made ereaders...