mattia - 6:09 am on Oct 16, 2011 (gmt 0) [edited by: lorax at 12:27 pm (utc) on Oct 16, 2011]
DO you know which keywords are used to come to you site?
if not, then this is where you need to start!
Configure and learn how to use Google Analytics to retrieve all the keywords used to come to your site.
Then, if your cart does not do it use a cart that will reveal to you all the keywords that do convert on your site.
if keywords used to come to your site are same before and after the Google Panda, then your conversion issue is obviously on your site (although I doubt this is the case).
my guess is that visitors are now coming to your site using different keywords.
[edited by: lorax at 12:27 pm (utc) on Oct 16, 2011]