All but 5 of my songs on iTunes disappeared?

I have never had this problem before, but the other day when I connected my iPhone to iTunes I noticed there were only 5 of my songs on iTunes. All of my other music is gone, but it shows up under the option "On this iPhone" when my phone is connected. I'm scared to sync my phone to my computer because it tells me that the music and files on my phone will be replaced with the library on this computer, which only has the 5 songs, and I don't want to lose my music on my phone too. This has never happened before, what can I do to fix it?

5 greatest songs ever