The iTunes Music Store keeps track of the status of your downloads and makes sure that you get every byte of each song and video you purchase. If a download is interrupted, the store keeps a note of that fact and makes sure that you have the opportunity to download all the tracks to which you're entitled.
Sign In to the iTunes Music Store
Choose Music Store from the Source pane in the iTunes window and connect to the store. If you are not already signed in, click the Sign In button in the upper-right corner and provide your Apple ID (or AOL screen name) and password to sign yourself in.
Check for Purchased Items
Choose Advanced, Check for Purchases. iTunes connects to the Music Store and checks the download queue for your account for any incompletely transferred items.
Note
About once per week, iTunes checks for purchased items automatically when you launch it.
Download Interrupted Tracks
If iTunes finds any tracks in the download queue that are still waiting to be retrieved, it begins to download them again automatically. Use the Purchased playlist in the Source pane to see which tracks are in the queue and are now being downloaded. (They're the ones shown in gray type.)