Thanks for reply. No, I'm not talking about comment spam. My webhost person, told me that your site is sending (Mod_security triggered) messages. It means that either your system contain virus or something else. I'm using my other wordpress sites from the same computer but nothing happened. It happen only when I try to access dashboard and the webhost server ban my ip automatically and after that I couldn't even browse the site. If I change the ip by restarting the broadband router, browsing of website again starts but as soon as I try to access dashboard, my ip got banned in webhost server. I asked the reason from the representator, he replied that your site or ip is sending too much spam. I'm using the other my wordpress site from the same ip and same system. This occur only with above mentioned website.
One thing more, this started after I upgraded to wordpress 3.4.2 from 3.4.1.