i have been working on my site for 2 months and start showing in google page 13 for my selected keyword. but suddenly couple of days ago it disappearing from first 100 pages of google. i am not sure what went wrong so please can somebody help me to bring my site back. thanks

As Denis correctly advises sites sometimes drop out of the index for a day or three. Usual advice is to wait and see - it could just be that Google is updating its different datacentres transferring up-to-date info around it network(s).

Of course if you have any reason to believe you may have *over optimised* (read: broken the web master guidelines or 'cheated') and your site disappears then you may wish to re-work your pages to remove your latest 'tweaks'.

Google webmaster tools area can be useful for identifying whether there are problems with your site - for example we accidentally barred Googlebot from our site a little while ago with an overzealouse tweak to a web security tool we develop. The Google webmaster web site quickly alerted us to the issue.

http://www.google.com/webmasters/tools/

You will need a google account and you will also need to go through a little validation process to prove you have control of that website. Basically Google ask you to add a meta tag to your homepage, or create a new text file on the server.  Once you have validated your ownership/control of the site you will have access to some useful feedback about what Google sees on your site and how it displays in the Search Engine Results Pages (SERPs).

Dean
http://www.deanmarshall.co.uk/