Can you tell us if all the music on the external drive is stored in one main folder or is it scattered all over the drive?
Can you locate the folder all your music is located in.
Once you find it make a note of the path so I will assume it is somenthing like G:\My Music
Next open iTunes and click file then choose add folder to library and browse to the folder all your music is contained within and then select the folder and click OK.
At that point iTunes should then scan the folder and add its contents to your iTunes library, so just let it sit and work and once it is finished you should then see all your music listed in the library so you can then use it.
The only downside is that you may have some duplicates so can you hold down the ctrl key and click on the duplicates you want to remove then hit the DEL key on your keyboard and then you will be prompted to remove them, Just click yes and then the duplicates will be removed.
If you have any issues let us know