iTunes completely screwed up!

Everytime I opened my iTunes, the Library messed up and refused to let me do anything except download the entire CD from an artist, which I did not want. It stopped giving my a song list below where I could listen to songs and purchase them individually. I tried rebooting, then came on here and tried updating it to iTunes5 or whatever it is called. The update completely wiped everything out even though it said it was an UPDATE, now I dont know where ANY of my music is that I loaded myself or that I PURCHASED! On top of that, the new iTunes keeps screwing up so it wont load. What do I do?