Lost my iPhone

i activated the lost mode but it’s offline

The "offline" message indicates that the phone has been powered off, the battery has run down on the phone, or the phone does not have an active Internet connection at its current location.

Unfortunately, there is nothing that you can do about this other than keep trying to see if the status of the phone has changed.

See this article.

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