i need to trace my iPhone & it is switch off

Your chances of tracing it while it is switched off are pretty slim.

If the phone has power, and Find My Network was turned on, and the thieves have not done anything to block radio signals getting to and from the phone, and another Apple device comes within Bluetooth range, that device might hear a Bluetooth signal from the powered-down, stolen phone, and relay a location.

This depends on another Apple device coming within Bluetooth range, so it's a lot less certain than the situation where the phone is turned on and has a Wi-Fi or cellular Internet connection that it can use to report its location.

Even if you see a location, it would not be a good idea to go there to confront criminals in person.

Also see:

If your iPhone or iPad was stolen - Apple Support

Do's and Don'ts for lost iPhones - Switched Off - Offline - Beware of Phishing – Apple Community

What to do if your phone is lost or stolen.

Find your lost iPhone or iPad - Apple Support