New iTunes lost all music

After attempting to download el captain, to which it says downloaded in APP Store window but won't allow me to proceed with installing the update(still running Yosemite), I believe the new ITUNES 12.4.1.6.

Next time opening iTunes, all of my library showed, but the text was light gray and the songs could not be played.

After a couple of toys iTunes no longer had any of my hundreds of records except for the most recent Bob Dylan record which I recently uploaded off a cd.

Then maybe a quarter of the music came back, for instance, it of 6 BECK records, it showed 1.

All of the music is still in my internal hard drive.

i checked Preferences/Advanced/iTunes Media Folder and the correct folder is picked.

Can anybody offer help?

Thanks

Chris