HT4527: iTunes: How to move your music to a new computerLearn about iTunes: How to move your music to a new computer
Currently Being ModeratedMar 6, 2013 1:34 PM (in response to Soccer Steph)
You should be able to copy iTunes purchases over via File > Transfer Purchases (File > Devices > Transfer Purchases if you have iTunes 11) - if you still have the 'pointers' to the tracks in your computer's iTunes library then you will need to delete them first, otherwise iTunes will see them, assume that you still have the tracks somewhere, and not copy them back.
For music from other sources (e.g. copied from CDs) you will need a third-party program on your computer to copy them back - there are some listed half-way down this page : https://discussions.apple.com/docs/DOC-3991