someone stole my iphone. when i signed in to find my... no devices apperared. how can i locate my stolen iphone

Assuming that you previously enabled Find My on the phone.......

The phone must have good power and an Internet connection to broadcast info about its location so that it will show up on a map.

If the battery has run down, or the phone does not currently have an Internet connection, it will not appear on a map.

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