Why has all my music has disappeared from itunes?

Itunes has been working fine for me for years but i recently bought some songs and now they are the only thing in my library. All my old music is on my ipod but i can't seem to sync it all back to by library on my pc. When i try to sync i have a message come up saying that if i sync i will loose everything on my ipod and i will be able to sync the few songs i just bought to my ipod.

Any ideas?