Q: Delete iTunes content and Rebuild

You can create a new library or re-store the old one..

Read how to create a new one here...

Open a different iTunes Library file or create a new one - Apple Support

Read how to restore one here...

If you don't see your entire library after you update iTunes on your Mac or PC - Apple Support

hauntvictim wrote:

I have moved my itunes library to an external hard drive, NAS.

How did you move it?

Unfortunately you did it the wrong way and doing it the right way would have taken 2 minutes.  For next time: 

Moving a library: Quick answer if you use iTunes' default preferences settings:  Copy the entire iTunes folder (and in doing so all its sub-folders and files) intact to the other drive, hold down the option/alt key (shift on Windows) while starting iTunes, and when requested to select a library select the iTunes Library.itl file in the copied folder.  https://support.apple.com/HT201596

The library file contains a list of the locations of files.  If you move the media folder the moving does not update this file and iTunes still looks for them in the old location. Changing preferences does not tell iTunes to change the location for old file references to the new place. The only thing that does that is either moving the entire library or to use iTunes itself to consolidate/organize media to the location designated in preferences so it tracks the move.