1) Try a different browser, if you haven't already.
2) Have you considered clearing your browser cache or tried a different browser? Your site may also have ended up in a disapproved area of your browser settings, so maybe consider clearing your settings to the default.
3) If all else fails, calling your host provider can help you sort this out, as your IP address may be flagged for some reason.
I know that in the past, I've helped someone with their AT&T U-verse service fix a problem like this, because it would intercept their web traffic in some cases and throw up its own proxy page. That was a hardware/firmware issue with AT&T's modem/router and they needed to fix it. I don't know if it ever got fixed, but I do know a lot of people had issues with it on the web. If you can try through a different device on the same network, and can get to the site, then you can likely rule this out.