Did you preserve your profile folder from the old computer? There are two main parts to an iTunes library, the media folder itself, and the database that records what content is in the library. I'm guessing you had created a split library, moving the media out to the external drive, but leaving the library files in the default location of User's Music\iTunes on the main hard drive. If you can recover this folder we can talk about how to link the two things back together properly, otherwise you will need to rebuild your library. If this needs to be done I suggest you take a look at my tip Make a split library portable and create your new library at X:\iTunes (where X: is the drive letter of the external) then move the media folder inside as X:\iTunes\iTunes Media before importing the media. If you have any Apple devices then see also Recover your iTunes library from your iPod or iOS device for tips on configuring the devices with a new library. There are links to scripts that can retrieve ratings, play counts, and playlists.
If on the other hand you had copied the whole library out to the external drive, then shift-start iTunes, click choose, and locate the .itl file on the external drive. Changing the media folder doesn't achieve the same ends.
tt2