I've had my Kindle for a few weeks, and I love it, but mine is a lemon. It locked up twice when it was in "sleep" mode (not turned off). I didn't realize I was rebooting it, but out of frustration, both times, I held the on/off switch to on for several seconds hoping for a reaction, and soon after I gave up, it just turned on.
This morning, my Kindle was not only locked in sleep, it also had crazy lines on the screen. After trying to get it to turn on..or off..I could see a faint message to call Kindle. The Kindle rep. was very polite and efficient. She said to slide the on/off button to on for 15 secs and then let go. Within 30 secs it should boot back up if it's going to. Well, mine didn't turn on this time, but that's how I happened to fix it before, except those times were just by chance. They're replacing my Kindle, and I should get the new one Monday. However, I was nearly finished with An Echo in the Bone, (about 50 pages left of a 820 page book), so I made a quick trip to the library to get the book. One thing I can say for books, they don't crash.