Go to your page's URL and review the error. If the error shows this:
The requested URL / was not found on this server.
Then you have likely deleted your public_html directory. You can remedy this simply by creating a folder called public_html.
If you go to your site and see an error looking something like this:
The requested URL /Site/Homepage.htm was not found on this server.
(note that /Site/Homepage.htm is simply an example and your error isn't likely to exactly mimc the above message)
Then you're likely working on iWeb. iWeb's index page redirects visitors to a specified page within your site. The page it's redirecting to doesn't exist, either because the Site folder (in this example) doesn't exist or because the Homepage.htm page (in this example) doesn't exist. Double check your files to ensure that you've uploaded all of the necessary files. If you need assistance uploading your iWeb site, please see our instructions here.