Iphone 7 locked

How can I reset my Iphone 7. I have charged it fully and put a new Sim card in it (the phone is not locked to a network).. The screen is displaying iPhone unavailable and in the bottom emergency. I have tried the volume and side button to rest but it does not work, it will accept incoming calls though. the pad lock in the top centre of the screen is in the lock position. Hoe can I resolve this.