No worries. If you purchased it from iTunes, then you can re-download it:
On the computer that you sync with:
1. Sign onto iTunes with the Apple ID you used to purchase the Music with
2. Make sure it is authorized - if it is not, authorize it under "Store" in the toolbar, Authorize this Computer
3. Access your purchased Music - Click on iTunes Store and open the Store
- Under Quick Links in the right-hand sidebar click on Purchased
- Click on the Music Tab
- Click on "Not On This Computer"
- Click on "All Songs"
Your purchased songs will be listed with a little Cloud with a down arrow next to each song
4. Download your purchased Music - you can click on each cloud selectively, or you can click on Download All at the bottom of the screen
5. Once your Music is downloaded to your computer, connect your device and select it from the sidebar to show the Device Profile Screens
6. Click on the Music tab and make sure you have checked Sync Music, and have selected the criteria for which music you want downloaded
7. Sync your devide and download your Music