incrediBILL - 7:16 am on Jan 8, 2012 (gmt 0)
I always start with the obvious, check my SEO/PPC first.
Did my traffic dip?
If my traffic slid then it's check the SERPs
If the traffic didn't slide, then is it the same keywords bringing traffic?
If none of this changed and it all looks fine, it's probably not SEO issues, then I start checking the competition, did they lower prices, introduce new products?
Also check to see if you have suddenly gotten some bad reviews, all it takes is one crazy customer that gets some traction in the search engine or on Yelp or somewhere to damage your reputation just enough to make a temporary glitch in sales.
There are also weather factors to consider, such as places where you get lots of sales suddenly having inclement weather such that causes major power outages, etc.
If you see nothing obvious, like the others said, poll the customers!