Some one replaced index page of my website. How it is possible? How i can prevent this to happen again?
Your site probably got hacked. Ask your hosting provider to run a virus/malware scan Scan your own computer if you got any virus/malware Change every password - make them secure - you don't know what data the hacker got http://www.makeuseof.com/tag/how-to-create-strong-password-that-you-can-remember-easily/ Code (markup):
Do you use the latest version for your script ? Ask the hosting company to take a look at the logs and tell you how this was possible.
I would suggest changing all the passwords associated with your hosting to start with (FTP, Database Users, Control Panel, Host's Billing Panel - if applicable). Once you have done that, I would suggest using a scanning tool such as ClamAV (if it's available) to check your site over.
It could also be the server that got hacked. Make sure your host is using up-to-date software, mod_security (Hopefully), and a well configured firewall. If it's your host, then the issue will just arise again. Contact your host about it.