is this -50 penalty?

last month all my keyword rank top 10 it been one month all my keyword around 500+ rank i check google webmaster tools i didnt receive any message please help me out what should i do build more backlinks or get new domain

This could be a penalty indeed. I have seen the same thing for one of my sites which was attacked and had porn pages placed in hidden directories in wordpress, it also contained links to other sites, hidden with javascript. So it was cloaking and received penalty.

You can sent me a PM with your site and I can check if the same thing is happening to you..

ok i sent you my site through pm thank you