"Offline" is displayed when one or more of the following are true:
- The battery has run down
- The phone was powered off
- The phone does not have a good Internet connection where it is located
Unfortunately, you have no control over any of these things, so all that you can do at this point is keep checking a few times each day to see if the status of the phone has changed.
Also keep in mind that iPhone thieves are very fond of placing a stolen phone is a special type of bag that will not allow the phone to either send or receive a signal......even if the battery is good and the phone is powered on, so that will also produce an "Offline" message when you try to locate the phone.
Keep trying for a few more days. If the phone is still "Offline" after that time, it's time to be realistic and accept the fact that the phone is gone.