I downloaded a song from iTunes but it doesn't work. I have tried updating software and restarted my computer. Can someone help me fix the problem?

Welcome to the Apple Community.

Try deleting the problematic file (electing to remove original file if/when prompted) and then re-downloading the file from the iTunes store.

You can re-download content purchased from the iTunes store (availability varies depending on location) using the purchased option from the Quick Links section in the top right corner of the iTunes homepage in your iTunes application on your computer.