The iTunes Library database file remains in the iTunes folder on the internal drive, even if you move the iTunes Media folder to an external drive. It should be at
<user home folder>/Music/iTunes/iTunes Library
If it's still there, you can try opening it using the method to switch between iTunes libraries.
http://support.apple.com/kb/HT1589
In the latest version of iTunes, the iTunes Library file seems to have an .itl extension. You need to use this procedure, not just double-click on the file.
If you want to create a new iTunes Library, the procedure is in the same document. To use your existing iTunes Media folder on the external drive, go to iTunes preferences Advanced tab. Where is says iTunes Media folder location, click Change to point it to its location on the external drive. Click OK.
I don't think this will make all of your existing songs appear in iTunes, because changing that location setting is going forward. If your iTunes Music library in iTunes is still empty, locate your iTunes Media folder on the external drive (using Windows or Finder on a Mac). Drag the entire folder to the iTunes window, and drop it there. Alternately, you can use the command
iTunes menu bar -> File -> Add to Library...
and select your iTunes Media folder. This should add all of your existing media. Unfortunately, supporting data such as playlists, ratings, playcount, etc. were stored in the iTunes Library file, so the new library will not have them.