Why are purchased and downloaded iTunes songs no longer playing on my iPhone in Canada?

Purchased and downloaded music (single songs and entire albums) from iTunes Store. they played for years and now they won't play anymore. Same goes for music I purchased from CD Baby as digital files. I can no longer play them from the Music app. in fact they are gone completely from my phone. I didn't delete them, they are just gone. I can't say when this happened, I just noticed that I was only hearing a steadly more limited selection of music when I played with Shuffle turned on. Same songs over and over. I used to have about 4000 songs. now I am down to about 400. Someone wrote that deleting the app and reinstalling would restore everything, nope, I lost everything (left with 10 albums - most recent purchases). I erased my phone and restored from yesterday's back up and that restored the 400 or so songs I have. but where are the other 3600? I bought them, paid for them. Do I get a refund from Apple?

[Re-Titled by Moderator]