Somebody has hacked into my google acccount and changed the password. Tell me how to regain my password ?

Click "Sign In" on google and then click "I cannot access my account"

From there you will have several options. Follow the one that best suits you :)

I have tried it,but couldn't success. Do you have any other idea ?

Well in that case your ONLY other course of action is to e-mail Google and explain what happened.

I Have Tried Everything , But Nothing Happened

Well if you have tried EVERYTHING then you are complete stuffed and this account is GONE forever.

So you have done EVERYTHING that is mentioned here:
Gmail: Help Center - I cannot access my account (http://mail.google.com/support/bin/answer.py?answer=46346&fpUrl=https%3A%2F%2Fwww.google.com%2Faccounts%2FFo rgotPasswd%3FfpOnly%3D1%26continue%3Dhttp%253A%252 F%252Fmail.google.com%252Fmail%252F%253Fui%253Dhtm l%2526zy%253Dl%26service%3Dmail&fuUrl=https%3A%2F%2Fwww.google.com%2Faccounts%2FFo rgotPasswd%3FfuOnly%3D1%26continue%3Dhttp%253A%252 F%252Fmail.google.com%252Fmail%252F%253Fui%253Dhtm l%2526zy%253Dl%26service%3Dmail&hl=en)

Enter your username and click "forgot my password", If you still remember your secret question answer it and you can change the password!

Benn, you obviously don't use Gmail, as there isn't a forgotten password link.
You have to follow the troubleshooting link I gave earlier.