Need to restore my music back to my iphone?

For some reason, my music on my iphone has disappeared. I don't know what I did. I have an iPhone 4 (not s, if that makes a difference.) How do I restore my music back from iTunes to my iphone.
10 points for best answer.

Hold both the lock and home button for 15 seconds you will get an apple logo and connect to iTunes, do so connecting your iPhone to your computer and restore it from your back- up

So you sync the music over from a computer? In that case open iTunes, connect the phone to the computer, click the 'music' tab at the top and make sure the music is actually syncing.