I am sorry to hear about your issue 😣. Have you tried to clear cache of My Files app? (Settings -> Apps -> My Files -> Storage -> Clear cache). I hope that helps!
Bizarre thing. Have you tried resetting your phone? But to change the website where you download these mp3 files? I used a website to download any song from YouTube for a while easily. But after a while, it started to work. I didn't understand the cause. We restart the phone but to no avail. I had told this to a friend of mine, and he suggested that I try to download the music from Ytmp3, where he downloads it. To my amazement, it worked. Yes, I had to download all the music again, but it's worth it. Now my music works, and I can enjoy listening to any song I want.