If the folder you deleted is the same folder you moved the your content should still be in it even if it isn't in the iTunes library. You can manually look for it and re-add it to itunes. If it is in a different folder that it must still be on your local drive. In that case just add them to iTunes.
My guess would be they are somewhere on the external drive and just need to be added to iTunes.
You can try to use the option key while launching iTunes again and navigate to Music/iTunes on your external drive just to make sure iTunes is looking in the right place for your library.
Either way I don't think your content is gone. If you only deleted what you copied then it is on the external drive. Otherwise it must still be on your local drive. You just have to find it. You can use Spotlight for that.