It's an "access denied" message, if it is just happening on one site that tends to suggest a problem with that particular website rather than your computer or browser. Have you tried accessing the site from a different browser or different device? Another possibility is that your IP address is being blocked by the website, you could try switching off your router, leaving it for a while and switching it back on (this should get you a different IP address - the longer you leave it off the more likely you will be to get a different IP when you switch it back on).