How do I locate a lost phone that won’t show on Find my iPhone?

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

KiltedTim is right... unless you had "Find My iPhone" enabled on the phone - when it is powered up in range of a wifi connection it will show up in https://www.icloud.com/find/

FWIW, my iPhone shows up about 50 feet from where it actually IS - so it is not like finding it in the couch cushions

You retrace your steps and look for it the old fashioned way.