Starting in iOS 9, we've seen a problem in which an iPhone, iPad, or iPod Touch gets stuck in what we call "the boot loop". While iOS is starting up on the iPhone, an error happens that causes the iPhone to reboot prematurely, ending up in an endless cycle of the iPhone rebooting. Sometimes you'll just see the Apple logo while this happens, sometimes you'll see the Apple logo with a loading bar underneath.
There are a handful of causes for these reboot cycles on the iPhone, depending on your own iPhone and backup history. Some jailbreak settings files may have stuck around in your backup, even though you haven't jailbroken since iOS 7, only to trigger a problem now. Or, you may have some settings stored in your backup that are only a problem now in iOS 9.3.4. Or, we've seen this happen when someone tries to restore a backup recovered with hard drive "undelete" style software, restoring files that have actually been overwritten with other data.
Depending on the circumstances of your specific backup, there are a couple of different solutions. If this happened "out of the blue," we usually recommend using Decipher Phone Refresh on your most recent iPhone backup, to create a backup with only your important data, and restoring that filtered backup. Sometimes, we also can pinpoint the problematic settings files by asking you some questions, and use a customized version of Decipher Backup Repair to filter out just the problematic files.
You can definitely contact us if you're experiencing this iPhone reboot loop and would like our advice on how to proceed.