kewords went away from 1st page to 50th pae page

[Karan Singh]

This could be because you might have done something wrong or it could be a Google dance, wait for sometime and continue with your SEO activity your site will be there.

[yinji]

maybe your back link is disappear,or you do not update your content.

[Hotter_DV]

if you keep history of your analysis on particular keyword then you can find the reason. If everything is fine then its happen to google dance or you loose any important link on particular keyword.

[jbennet]

Well if they are on the 50th page at least you're still in the index so it's not all that bad. Has there been any recent changes to your site? Has there been any link building changes?

[Webby49]

Use a tool like opensiteexplorer.org to monitor all your backlinks. A drop this drastic indicates that Google has probably penalised you possibly for some SEO practice which is against their rules. Their rules are very comprehensive so it's possibly you broke them without knowing it.

Have you paid for any links on external sites which have keyword anchor text? They are really cracking down on this. Also, if you have been blog / forum commenting and your comment went in a 'latest comments' module on every page of that site and added a few hundred links in a day that could be seen as spam. Best to retrace your steps, analyse your links, check your keyword density and see how you may have overstepped a very fine line between what's seen as good SEO and spam.