some keywords of my site are rank down suddenly , I am trying to find the answer of this question why this happen .can you guys suggest men something about this.

This may be due to Google's algorithm update. They must have updated their search engine algorithm. To avoid such things in future, have relevent and fresh content on your website.

change the content your site or remove flag or spammy backlink from your site.

Even my website is also suffering with same problem all my keywords dropped from first page to 50 pages :( dont know what to do..

I am sure this is due to Google panda.You website has surely backlinks on dead site or useless site.Try to good resources.Never make backlinks on bad sites.

This down fall in keyword ranking because of the Google penguing.Penguin hit on the quality of links porinting to your site. Check the quality of your links that you have earned for your site.