How to fix unexpected shutdown on my phone?
How to fix my phone?problem is unexpected shutdown
iPhone 6, iOS 12
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See this Apple support article—>If your iPhone, iPad or iPod touch won't turn on or is frozen – Apple Support
After it starts go to Settings/General/iPhone storage. If it has less than 1 GB free that may be the cause of the problem.
Also check Settings/Battery - Battery Health. If it is below 80% or displays a message saying the battery is not working well the battery needs to be replaced, and it can also cause this problem. Apple will replace the battery for $49.
If this is a recurring problem and is neither of the above your phone may have a hardware problem. A phone that old probably isn’t worth repairing.