Depending upon what country that you are in then you might be able to re-download on your computer's iTunes via the Purchased link under Quick Links on the right-hand side of the iTunes store home page, and, if it's an iPod Touch, the Purchased tab iTunes store app on it : re-downloading.
If you can't re-download it then connect the iPad to your computer's iTunes and do File > Transfer Purchases (File > Devices > Transfer Purchases if you have iTunes 11 on your computer), and you can then sync the music to your iPod.