i lost my i phone & find my i phone saying device is offline

If it is offline there is nothing you can do. Find my iphone can only report a location if the device is powered on and connected to the internet. If it is offline, then there is no remote tracking possible.

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

See these articles https://support.apple.com/kb/PH2698?locale=en_US

https://support.apple.com/en-in/guide/iphone/iph09b087eda/ios