I put all my music on a playlist seperate from the rest of the music on the iTunes library, so I could easily get to what music I wanted to listen to, instead of my brother or sister's music. My brother recently got an iPod nano, and didn't know how to use iTunes, after explaining it to him, he deleted my whole entire playlist. I didn't know, so when I hooked my phone up to the computer to sync some songs I was going to buy, all the music on my phone disappeared. I've tried re-doing the playlist and syncing all of it to my phone again, but it won't work. I've also tried turning off my phone for certain periods of time, still nothing. I lost about 200 songs (thankfully not more), and I don't know how to get it back.
If your brother deleted the songs themselves they are gone unless your computer is backed up.
If your brother just deleted the playlist you need to recreate the playlist and then re-sync it. Here is a how to article on how to do that. - http://www.ehow.com/how_2214496_sync-playlists-iphone.html