What was the message that appeared when you logged on to iCloud to try to locate your phone?
"No Device Found".......This means that Find My was not enabled on the phone that is lost, so it cannot be located online. You have to look for the phone the old fashioned way......with your eyes.
"Offline"......This can mean different things. 1) The phone does not have an Internet connection where it is currently located, 2) The battery has run down, or 3) a thief has wrapped aluminum foil around the phone or placed it in a Faraday bag so the phone cannot send or receive a signal.
All that you can do if the phone is "Offline" is keep checking a few times each day to see if the status of the phone might have changed.