If you have already tried adjusting the volume of your phone (using the physical buttons on the upper right side of the phone), you may check if "quiet hours" is active. You should see a crescent moon icon near the top left side of your screen when "quiet hours" is active. You can toggle this feature in Settings > quiet hours.
Quiet Hours is a feature that is usually automatically enabled when you set appointments in your calendar and mark your status as "busy", or when you choose to prevent your phone from disturbing your, e.g. while you're sleeping at night.