MacRumors notes that complaints about this problem have been spotted on Reddit, Apple support forums and the blog’s own forum pages, so it sounds like this is a reasonably widespread problem. In a nutshell, users have found that their devices shut down completely for no reason, and that only holding down the power button and home button for 10 seconds will get their phones to restart.
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Interestingly, it seems that there have been some complaints about this from iPhone 6 owners as well, meaning this might be a problem with iOS 9. Users have also reported that the home button feels unusually warm when trying to reset their devices, suggesting this could also be an overheating issue. All the same, there isn’t enough information to pinpoint what the problem actually is just yet.
If you’ve experienced this issue on your iPhone 6 or iPhone 6s, you can help MacRumors figure out the causes of the problem by adding your experience to its data pool over at this forum thread. Hopefully it’s a software issue and Apple can get this problem solved before it releases the next version of iOS 9.