Log on to your website with FTP and look for the .htaccess file. You will probably see some garbage in there that you need to remove because someone hacked your website and changed it to route traffic to their site instead.
I would also notify your webhost, its always good to tell them. They know this happens though.
Also look for some PHP sites dropped on your site, check your folders ... delete all those garbage PHP pages they put on your site also. Change your password also just to be more safe.