All of mine lately have been from Words With Friends as well., It will work perfectly fine, and then randomly lock up. I hope a fix is coming
I had the exact same experience with setting an alarm only for it to lock on my overnight and I was late for work! I've been working with Nokia Support to try to narrow down the cause. So far, I've been able to recreate the 'locked phone' phenomenon with a freshly factory-hardreset phone with ONLY Google+Microsoft accounts setup and Whatsapp installed. So yesterday I deleted Whatsapp. So far it's only been 14hours but NO lockups so far, so we shall see. If there are no lockups by 48hrs I would be willing to guess Whatsapp is to blame. Prior to this, I was experiencing at least one lockup/day. I notice that Whatsapp was taken down from the Windowsphone Marketplace yesterday and is no longer available. Coincidence?
This happened to me when I was literally doing nothing with the phone at the end of the day - it was just sitting on my desk, dark, and wouldn't turn back on. I thought maybe the battery was dead, so I plugged it in and went to bed. However, it appears that the device doesn't charge when it's in this state.
The soft reboot (power+volume-down) worked, though. It's hard to know if this is a hardware or software problem, but I'm going to file a bug with the Windows Phone team (I work at MS) & refer them to this forum, so if you're having this problem, please be as detailed as possible in describing any observations you many have.