Don't get too worried if its only been happening a day. If it stays like that for a week, then you can worry. Google plays with their placement all the time.
My only guess is that your site got sandboxed. It might take a while before your site rank again in google. One of my friend also got her site sandboxed by google. But it won't take a very long time. Azim
This is interesting because the same thing happened to my blog recently. I've lost all ranking for all keywords I was formally on the first page, if not # 1 for. My blog is a blogger blog that has been #1 for several keywords for years. Over the last few months, it started slipping so I used 30 minute backlinks (30minutebacklinks . com) to regain some ranking. It seems a few days after using this method, my site totally dropped from all keywords I was ranked for. I don't even know who to contact or how to undo this, but I'm hoping the site comes back soon.
hi i am facing the same situation like you right now i ranked well 1st page for most of my keywords yesterday, the SERP of all my main keywords suddenly drop from 1st to page 6