Hi Friends,
Usually they will tell you, e.g. if some kind of attack mitigation is blocking you. They may display a captcha instead or replace the web page with a non-functional one. In this last case you cannot directly know if you are blocked unless you know how it looked before, but you can try to access the same website with a proxy or vpn service (tor, hidemyass etc) or try to create a snapshot of the page with e.g. Archive.is
If you notice a webpage suddenly changing their appearance, it might be worthwhile to delete cookies, as they may not really have blocked your ip but set a cookie indicating you as a bad guy
Go to Internet Options in Control Panel and on the Security tab, click on Restricted Websites in the Internet Security Zone, and then on the button labeled “Sites” (See image below). Check if the URL of the website you wish to access is listed there. If yes, select the URL and click Remove.