My iPHONE 4 goes completely dead. The Battery is good. This is the second time this has happened and I do not know how to get the phone back on.

My iPHONE 4 goes completely dead. The Battery is good. This is the second time this has happened and I do not know how to get the phone back on. I just had my son call the phone, it rings for 40 seconds and then it does go to voice mail he said. But I am looking at a black screen, I tried pushing the button at the top,but the screen is just black. Does this have to do with the new OS 5 that was installed and how can we get the phone to work again.