I recently had a very odd problem with my Kindle (3rd generation). I had it connected to my computer, ejected it from my computer, and when the screen refreshed there were just the power and wireless icons in the top-right corner. No listing of items and the buttons didn’t respond in any way. When I slid the power button, it flashed green for a moment and turned out.
Long story short, it took a full reboot to bring it out its little problem.
In order to reboot the Kindle, slide the power switch to the on position, hold it there for 15 seconds, and then release it. Wait several seconds and you should see the Kindle splash screen and a progress bar indicating that it is loading. It takes a few minutes to load and then you should be good to go after that.
Note: This procedure appears to work for the generation 2 Kindle and the Kindle DX (the 9″ version).