Did your catalog load and open? Or did the new version start with and present a default catalog (which would be, of course, empty)?
Try navigating to your old catalog, which would have been converted into the new database format, and see if it will open. It should have all of your images from before.
Without more information, my first thought would be that you didn't open your actual catalog, but an empty one created by LR (which it's what LR does if it couldn't find your catalog).
Exactly. It is a bit shocking when LR does this, and oft times it will do it without warning you that it cannot find the old catalog, especially during an upgrade when LR can forget about many preferences.
"Migrated" sounds like you are using the new "Lightroom CC" and migrated a Lightroom catalogue. If Lightroom appears empty, maybe you picked the wrong catalogue to migrate?
But if you want help, you have to give much more information.
