Find my iPhone is offline how can I find the last location it was at ?

When the device is turned on, you will know where it is -- If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support.

You can only locate, lock or erase your device when it is logged into iCloud and 'Find My Phone' is enabled, additionally the device will need to be switched on and connected to a wifi or cellular network.

Unfortunately, you cannot activate iCloud or 'Find My Phone' remotely.