The xref'd file has to exist, of course.
If it gets moved, all that will appear is a line of text containing its last known path.
If you are not viewing the insertion point perpendicular to the UCS, and the file has been moved or is planar, you may not be able to see it.
It's possible that you're zoomed out so far that you can't see the file or its link, or zoomed in so cloae that you can't see it either.
So, zoom extents. If that doesn't make it visible, try orbiting.
Mike Halloran
Pembroke Pines, FL, USA