Lost purchased music after update

Hi,

I updated my iphone just last week after 11 months of not updating. I have just noticed that since the update I have lost music that I have purchased from itunes. I don't know how much music is missing, however it seems to be quite a bit of recently purchased music. How can I retrieve this music? When I go onto iTunes and look at songs I know I have purchased it says "buy now" not "install" or "download" so it looks like it will charge me again of I try. I won't be impressed at all if I have to purchase all the music I'm missing again. Not only that, I don't know how much music is missing.

Please tell me there is a way to get it all back!