i lost my iphone to thieves and it says offline on my find my iphone

Start here:

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

Offline means that the phone is powered off, the battery has run down, or the phone does not have an Internet connection at the time.

All that you can do is follow the instructions in the article posted by IdrisSeabright above and hope that the phone might come back online so that you can see the location it on a map.