Iphone 7 black screen and not working

My phone screen suddenly went black. I couldn't turn it off and the home button wasn't pushable. Eventually it turned off and eventually it came back on (after I'd been holding the on button). Now the screen just had the apple logo on it. But again it doesn't respond, again home button can't be pushed in and holding both the volume down button and the on/off button for 20 seconds to link it to Itunes, doesn't work either. Any help would be appreciated. thanks