My iPhone 6s randomly shuts off.

My iphone 6s randomly shut off. I went to double click the home button to swipe away an app but after I double tapped the home button my iphone because unresponsive and I couldn't even turn the phone off. Then it randomly shut off and now when I try to turn it on it goes straight to the apple logo then goes to the darkish screen then turns off completely. I have tried pressing the power button and home button, I've also tried the power button and both volume buttons but it continues to do the same thing. I also tried the iTunes thing but I have a chromebook and it didnt work. Can someone please help me? It's really stressing me out and I dont want to take it to the apple store and have to spend alot of money to get it fixed.

[Re-Titled by Moderator]