Can't download previous purchases in iTunes
You could rule out the possibility of a local problem by attempting to download purchases on another authorised device, and refine the diagnosis by trying it both on and off that same wireless network. 

Or you could head straight for iTunes support - when I got corrupted characters in my pre-purchased copy of the Steve Jobs biography last year they had a fix to me within two hours. Based on the Q&amp;A so far I think you're going to need to get Apple to look at your account.