I can't imagine how something like this might happen, unless you, someone who uses your computer, someone who administers your network, or your ISP has blocked imageshack.us. Can you access the imageshack website itself? If you can't, you might have to do a little digging to see who might have blocked the website.
Aside from that, I can only recommend other free image hosts like Photobucket or Tinypic. (or http://www.mylionking.com/sigs for your personal sigs).
Sorry I couldn't be much help!
