Why does my phone randomly shut off and...

My Iphone 6 will randomly shut off Even when the the battery is not depleted and the only way that I can turn it back on is by holding both the home button and the Sleep / wake button at the same time. (so i basically have to hard reset it everytime) I also have the greyed out wifi issue so I dont know if these two problems correlate at all.

My warrenty is up and the phone is no longer being used on a carrier contract so I only wanted to use the device like a ipod on wifi.

I was almost positive it wasn't the battery because when I would turn it back on it would still be at the same percent. But just yesterday I charged the phone to 100% and didnt use it all day/night and when I went to use it this morning the charging symbol that indicates the phone is dead popped up.

My head is spinning because I know when I initially turned on the phone to start charging it that once I stopped using it it was just going to turn off again on its own so I don't know how the battery could have been completely dead when the phone was off and charging all night.

I believe the phone won't shut off on its own as long as I'm using it but I haven't really tested it out yet.

Any help please