I cannot find my phone someone take it and offline

For it to be online it must e powered up and connected to the internet over WiFi or a cellular data connection. So if someone turned it off, if the battery died, if it is unable to connect to the internet, or if someone erased it in iTunes then it will appear as offline. There is no way for you to remotely bring it online.

You will never be able to track it as long as it is offline.