How can i stop my iPhone 6 from unexpected shut down

Please follow the steps in the following link for support with this issue: If your iOS device restarts or displays the Apple logo or a spinning gear unexpectedly

Hi,

Do a force restart your phone by pressing sleep/wake button together with home button until apple logo appears in the screen.

If it not solve your problem, check your phone battery with apple or its authorized service centers. It may be aged.