Welcome to Android Central! Check to see if the Camera app somehow got Disabled. Go to Settings>Apps, and look for Camera. If it was Disabled, it will either be in a separate "Disabled" column next to the "All" column, or it will be at the very bottom of the "All" column. If it was Disabled at some point, just select it and tap Enable.
Otherwise, it might be a hardware issue. When you say you reset the phone, do you mean a full factory data reset, the type that wipes your phone clean?