Something happened to my device yesterday, thought I'd share in case it helped anyone else. While listening to a podcast (using gPodder), the device locked up; screen contents were not changing and the device was not responding. I tried to force reboot it by holding the power button for twenty seconds, but that did nothing. Holding it for a minute caused the screen to turn black, however the phone was apparently still "on"; it made SFOS sounds on receiving emails. Further attempts at holding the power button caused the device to vibrate briefly at ten seconds as though turning off or on, but nothing happened. The solution turned out to be holding power+volume up for six seconds, which is a hardware interrupt to force reboot the phone (simply holding power is apparently a software interrupt).