Can’t turn IPhoneX off when holding down volume and Siri button. iPhone Xwont open any apps.

My Dads IPhone X will not open any apps, we even tried asking Siri and her response was ‘Sorry I cannot open settings for you’- the same applies for every other app. He is able to swipe across the home screen however can not open any apps e.g. messages, email, safari. We hen thought that turning off the phone would fix the problem, however that’s not the case. When holding down the volume button and ‘siri’ Button on the right hand side of the phone, the ‘turn off’ screen does not come up. Therefore we are unable to turn off the phone. This is a new phone, only being bought at Christmas so I’m not sure what the problem is. Please help!! Many thanks.