How to get the music I purchased with itunes back?

I regularly purchase music on iTunes and have done so since it began.

A subscription to Apple Music for a month or two reminded me to avoid subscriptions. These are not for me.

I have also changed my login to Apple to a new email address.

What has happened?

My iTunes library has been decimated.

I can't listen to my music on the iPhone or the computer. Something has stolen my music. Music that I purchased, paid for, and treasured. I want my music back.

What do I have to do? I am not unsubscribing to Apple Music.

I feel this is so unjust. I am so upset and angry- and sad.

Asking for help here -for something that should not have happened- seems the only solution.

When I go to the 'Purchased' list in Itunes, and select an item, the message

" The song could not be used because the original file could not be found. Would you like to locate it?" appears. The files in that folder are empty. How can I download them all again and protect them against loss?

Thank you.