It’s not you. It’s Twitter. Twitter.com has gone down, though the mobile version of the site, as well as access to Twitter through Twitter clients seems fine.
The Twitter status page confirms the problem, as posted at 5:03pm ET:
Tweets flowing through Twitter for Mac for me continue just fine. I’m also able to reach the mobile version of Twitter.
Chances are, this will be a temporary problem. We’ll update, if we hear more.
Postscript (5:22pm ET): And we’re back, with Twitter.com appearing to work normally now.