I have experienced this multiple times, and it seems to always be worse if I have locked the screen. Yesterday I was going to walk away for awhile, so I turned off the wireless card, locked the screen, then closed the unit up. When I came back, it was totally unresponsive on the LCD side. The power button was still lit up, the trackball was lit up, and interestingly, the ereader side was still totally functional. I just could NOT get the LCD to reactivate. Here's what I tried: I closed and reopened the unit. I pressed, re-pressed, and held the Menu button. I pressed all of the other buttons too. I folded the screens back-to-back. I plugged in and unplugged the power cord. Finally, I held the power button in to force it to turn off. That did not work....it would not shut down. Through all of this the e-reader was happily chugging along. Finally, there was no other option but to remove the battery.
This has happened to me multiple times. Sometimes I can shut it down with the button, but usually it requires pulling the battery. And this leads me to a major gripe. The battery is INSANE to try to get out! Whoever came up with this design gets an award for "End User Frustration". Once or twice I've been able to turn it up on edge to let gravity assist in removing it. But that doesn't seem to work well anymore. I have to exert inward pressure on the release buttons, which holds the battery into the unit. The last couple times I have had to resort to releasing the catch on one end while using a thin knife blade to pull that end outward as far as possible. Then release the catch on the other end and it finally pops out. It would help to be an octopus during this operation though.
Obviously, no thought was given to the possibility of having to do this routinely.

This lock-out problem NEEDS TO BE FIXED, and FAST. This is more critical than adding functionality at this point.