rebuild itunes library after crash
You have two options:


restore the library from a backup copy (previous libraries folder in the iTunes folder, Time Machine, other backups)
re-import the songs by dragging the media into the library section and allowing it to add the tracks back to the library - optionally you can import any playlists or library xml documents that saved the playlists, ratings and data relating to the tracks and not embedded into the media files on the drive


If you have enabled iTunes Match, you will also be safe from most data loss since all playlists are stored in the cloud unless it contains exclusively tracks that were not matched to the cloud like podcasts and other oddities like voice memos and low bit-rate recordings.