First thing you should do is try to delete all cookies related to the forum. I don't have IE9 but in IE8 you need to go to Tools > General > Browsing History > Delete > Check "cookies" and click "delete". I'm assuming you can find more detailed instructions online.
Another thing to note is there are several available domains - forums.xkcd.com, fora.xkcd.com and echochamer.me. If a link in a post directs you to one of these domains but you're not logged in to it, you may need to log in again. This doesn't sound like the current problem, though. Hopefully cookies will be enough to fix it.
TaintedDeity: The trick to being a good poster is in the posts you don't make, not the posts you do make. Or something.