Start with site:mysite.com, check for malware, check Webmaster Tools. Try the Webmaster Forum. Is it just your site, or lots of sites?
Copy and Paste this code to your site:
The Short Cutts Guide To Penguin 2.0
Copyright 2013 © Click Consult
And what you can do to improve your site's performance
Boost your revenue with our digital marketing tips
Linking strategies for a clean, organic website