Iphone 6s won't start up. Stays on apple logo. Tried restart and doesn't work

(1) Try and Force ReStart Your iPhone as instructed in this article by Apple ➔ How to Force ReStart your iPhone. If that does not work then -

(2)  If the above does not work then - Follow the instructions in this article by Apple ➔ If your iPhone is Non-Responsive or Frozen and see if that helps.

(3)  If that does not help your phone might need service ➔  Contact Apple for support and service - Apple Support

Axel F.

Mine did this too. It was just not enough memory. I plugged it into the charger and left it alone for days and now it works but now I have two iPhones

You are welcome and Good Luck.

Axel F.