Chances are your .htaccess file has been compromised.
If this is a WordPress site you could have a plugin that allowed access or a breached username and password. The .htaccess file is a hidden file in the root of your website. Open the file and you will most likely find a few forwarding commands in the file. Delete them and that should stop the forwards. Make sure to backup the file in case you remove something you should not.
When using wordpress this can happen through a malicious plugin or theme. Make sure to update wordpress to the latest version and keep plugins and themes updated.