First thing to do, forget the stats - go on your site and BUY SOMETHING, not just go through the motions. Pay with your personal PayPal account and just refund it later.
After you've done that, do it again in another web browser. Didn't IE release a new browser about 3 weeks ago...?
My initial thought is that if you were only making one sale a day, it's possible that one sale was coming from one place - maybe a blog that mentioned your product, but didn't link to it, so people would go and Google it to find you. And now that post is buried, or was kicked out of the search engines with Google's most recent content update. Could be a lot of reasons there... According to your analytics, where were your SALES coming from? Forget about your traffic, look at the people who actually bought something and figure out where THEY came from. Then see if traffic from THAT source has tapered off.