Hi Cory, thanks for your reply
Q-Do you have a Time Machine or other backup of your data from before the update?
A- I did have a backup on an external hard drive but the drive developed a problem a year or so ago, would not back up and I could not fix the coded error (numerous times) so lost interest in / forgot about it.
Q-Is the Music Library file still the same size?
A- There should be around 30GB of music but it is nowhere to be found, either on the iMac hard disk or in iCloud. The only music available is music I have downloaded from the Apple Music Store, barely a GB.
Q-Are the songs still in the physical ~/Music/Music/Media folder?
No, only a few MB
The odd thing is that my 30GB 4th Gen iPod and iPhone 6 (which I don't use but keep as a spare) have all the music on them and they are backed up to iTunes every few months. Unfortunately, I could never work out how to import all of my music to my daily driver (iPhone XR 128GB), although the purchased Apple Store music syncs with that one ok.Could it be that I only have a 50 GB iCloud account and it is full?