Hi:

Do you have Google Analytics installed, including the conversion function. If not install it. Next set up a funnel for the total sale cart function. See if the sale funnel is dropping off a some point that normally it would not.

If you are really concerned you might use ClickTracks instead of GA. ClickTracks is log based and this way you can check all your stats before and after your dramatic sales drop.

I am assuming that your search/referrer traffic really hasn't dropped. But the analytics can help with this too.

Good luck.