I still havent been able to sort this yet.
I originally got the message about cant load content. Basically all purchases from Apple (95 songs).
After 5 hours with Apple support, no solution. I found the music in itunes folder on computer, copied and reimported. Then I deleted all the originals that were duplicated (itunes view showed the number of plays so if that was blank I counted it as the new file). This worked. These previously gave message like 'content no authorised' or 'cannot be decoded'
Unfortunately for me, next day the other 400 songs I had uploaded and previously worked are now greyed out. They show in the music app but cant play. New message for those not working 'The song is not currently available in your country / region'. Note, some of these songs have been in my account since 2012.
There is an update to Itunes vers 12.13. I was having a problem before updating so I'm not sure if its to blame. You can update by using Apple Software Updater on Windows PC or if you go into Microsoft store it identifes the new version and will update. both seem to work.
My next step is to delete 20 songs that are greyed out and to upload them again to see if that works.
I'm not sure if that helps but after 2 sessions (5 hours and 3 hours) with Apple support on phone I wasn't getting anywhere so I've worked through a few things