Itunes normally stores files in the following path
/Music/Itunes/Itunes Media Folder/Music/Artist
So if your music files are still on the machine you can import it agian either using drag and drop or with File, Add to library where you can browse to the location.
If you want to use Drag and Drop the View, Show Sidebar may help you see things as was the norm in itunes 10
Thank you, Steve. I found them. The path in my iMac is /Music/iTunes/iTunes Music . Therein are listed all the artists represented in my library and, for each artist, all his or her songs.
Now, wouldn't it have been nicer if iTunes Help explained this?
Apple, where inadequate end user documentation is not a problem, it's a way of life!