Depending on which iPhone you have, your smartphone will have two volume buttons, a side button, a Ring/Silent switch, or an Action button, and if it's an iPhone SE or older iPhone, a Home button.

Next up is the action button, up on the left-hand side as you look at the iPhone, which has replaced the old ring / silent ...

Restarting your iPhone is usually straightforward, but it gets a bit trickier if you don't have a working power button.