It's hard to know for sure without more information.

Google recently released a new algorithm update: Search quality highlights: 40 changes for February - Inside Search

Do you have any messages in webmaster tools? Is your site still indexed? Were you engaged in any shady link building practices?

As i keep saying to people, try not to worry, especially if you have a website that isn't thin, has high quality content and you use a backlinking method that isn't obvious manipulation. Keep calm, wait a few weeks, maybe months - you will probably see it come back.

However you may want to ensure that there aren't obvious issues causing these problems.

I wouldn't worry too much. It sounds like the Google dance.

Keep doing what you're doing and you should come back on top soon.

The only other thing I can think of is your site may not meet Google's quality guidelines. Make sure your site is not overloaded with ads and that it contains good content for the users.

It could be just something temporary, like the Google dance phenomena.

google already update it algo in this day.. just wait about new changed fromm google

Start building backlinks constantly without using ANY blackhat method and after a few days I am sure you will get your website back at 1st page

Keep adding original content and build some high quality backlinks. Google updates it index each day so be patient.