I switched on my Mac and went into the library and there was only one recent purchase showing from the iTunes store.
About 35 GB of music had just gone. Not to worry I had the trusty old iMac Time Machine App to get it all back. Going back a couple of days there it all was so I just clicked "Restore" and there was evidence that the files were being copied back.
On completion still no library. With panic setting in I tried it all again but same result.
The third time I went back in the Time Machine I noticed that the iTunes Media Library had moved in the Folder Structure.
I had recently installed a Synology NAS DS214Play as my own Network Server, ("Build your own Cloud"). Not fully understanding the Synology Apps I had selected one that controls your ITunes Library but couldn't seem to get it to work.
It was this that had moved the iTunes location as below.
Notice it is under the Main Folder ....USER(your user name)_Music-iTunes-ITunes Media-Music-then the tracks are listed.
This did not populate the Library in iTunes
This is how it should look. Just Drag the Music Folder from Column 2 into Column 1 on to the Music symbol and the tree structure is correct again.
I am not sure why the Time Machine restore didn't do that but instantly your Library re-appears
I hope this helps someone who has the same problem