First I understand you host the website some where else. However to answer your question you
The first solution: You need to loging into the cpanel, go to filemanager, when i opens, click on public_html when it open you will locate Index.html , right click the file and click delete, it will be deleted from the site.
Second solution: it means that your password is incorrect, make sure you type the right password ask, if you type password provided to you correct, request for password reset from the hosting provider.