If you already synched you iPod, then you have to buy them all back.
If you have not yet synched your iPod, then there is an easy way to get all your songs back!
http://www.mediafour.com/products/xp...ial/survey.asp - Use this program (download the free trial) and use it to copy songs off of your iPod and paste them onto your iTunes music folder.
I used it not too long ago when our computer crashed. It worked really well! The only setback is you can only copy over five songs at a time, so it's very time consuming, but it's well worth it.