Puke: You don't know where your towel is? Oh dear.
I have a friend who also fell foul of this inexplicable error doownloading the game. In his case he aborted the download manually in order to restart it with a more reliable ftp client, and was told falsely that he had already downloaded the file
Go to the support page and make sure you generate an official support ticket -- they dealt with it very promptly once he had done that.
My question to Shrapnel is... why on earth are you adding this unnecessary flaw to your system? If someone has bought the game, there is no reason to prevent them from downloading it again, regardless of whether they completed the download the first time.
The present situation, where it locks you out if the connection dies for any reason (and I'd probably run out of fingers listing possible ways in which this could happen), really isn't acceptable.
I realise the system is new, so I just hope that resolving this is fairly high on your list of things to improve.