Samsung s4 isn`t turning on at all?

I must say that I was excited to have my new phone (actually it isn`t new phone but it was nicely used so it was almost new)

and I downloaded everything I liked . it suddenly switched off on my first day and I was surprised why it suddenly switched off even though the battery percentage was 68%.I tried switching it on and it was okay but on my second day when I was watching video on youtube it suddenly switched off again even though the battery 33% percentage. And I still can`t switch it on. I charged my battery almost 2 hours and still it isn`t turning on at all . I`ve tried everything(searching through internet and still haven`t succeeded , such as pull out the battery ,tried to reboot ( volume up , turn on\off , home buttom key ) . I really don`t know what to do at all please help me please . I will really appreciate it. Sorry for my English