Gemini23 - 9:18 pm on Jun 2, 2012 (gmt 0)
it sounds like Penguin.. did it hit on April 24th/ April 25th?
If Penguin then the likelihood is your link profile needs some work... remove spam backlinks.. and hopefully have some more 'good links'.. :)
You need to email or contact websites where your spam links are.. keep a record of everything you do...
You could also head over to the Google forum where you could post your url... and have others comment..