Why did I lose 350 of my iTunes songs, and how can I get them back?

I synched my iPhone the other day, before downloading the new 4 update. I lost 350 songs. I still have about 500. I downloaded the 4 update and resynched the phone. I still didn't have the songs. At the suggestion of Apple, I removed iTunes from my computer and reinstalled it, and then synched the phone again. I still don't have the songs.

I tried to log onto iTunes online, but I couldn't. Apple said the store was down but would be up in a half hour. That was six days ago and I still cannot log on.

Can anyone help?

(The nearest Apple store is a half an hour away, and you have to make an appointment to visit the store. I would like to avoid this if possible.)