What app did you originally download the music in ? You can only potentially redownload iTunes purchased music via the Purchased tab in the iTunes store app - or if you've subscribed to iTunes Match on your computer's iTunes and uploaded your own music to it then you should also be able to download/stream your one music.
But if the music came from a different source and you haven't subscribed to Match then it won't show in the iTunes store app for redownloading. If you have the music in your computer's iTunes library then you should be able to sync it from your computer to your iPad : connect the iPad to your computer and select it on the left-hand sidebar of your computer's iTunes (option-command-S on a Mac, control-S on a PC, should get the sidebar to show), and then use the Music tab on the right-hand side of the iTunes screen to select and sync it to the iPad's Music app.