First change your back end password. Make sure your new password consists of a combination of letters, digits and special characters.
Google has a guide page to recover from hacked state, check: http://www.google.com/webmasters/hacked/
Please mention the type of the site (I mean wordpress, joomla like CMS or static page site?). The techniques to recover the site will vary depending on the site's design.
Please contact your webhosting. They will help you. Don't forgot to ask log details. And please be sure if you are running joomla1.0 , update it( joomla totally ends the support for 1.0). Some of the webhosting company take the backup daily. Ask them.