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Short answer: Find out what specific part of the boot sequence fails and, if needed, 'google' the problem
The first thing to try would be rebooting in "Safe Mode" which cleans some cache and only loads the minimum from the system to operate.
If that doesn't work, then you'll need to reboot the computer in "Verbose Mode", see where the login procedure is stuck and Google the problem
Does your iPhone 6 suddenly shut down for no apparent reason?
This issue is coming from a bug that appeared after upgrading to iOS 10.
Both the iPhone 6 and the 6S have been hit but only the latter is subject to the official battery replacement program from Apple.
As a runner, I’ve experienced this problem and the colder outside, the faster the iPhone would shut down unexpectedly, starting at around 50% charge.
Then, to get it back on again, it could take between a few minutes to sometimes 10 minutes (which I suspect is linked to the charge level and the temperature of the battery).
