That message is usually generated when there's a problem with the coding of the site itself (most commonly if it's been designed using WordPress'). If that's what's happening in your case, there's nothing you can do about it, as the problem is 'at the other end'.
However it might be worth clearing your browser's cache and/or trying a different browser, as that can sometimes fix problems with accessing certain sites. (If you need instructions for clearing the cache, please state which web browser you're using).
It's also worth remembering that what appears to be a single website is commonly a collection of three parallel sites these days, with one site automatically loading when you visit the web address on a PC or laptop, a different one loading when you use a tablet and a third version being displayed on mobile phones. So, if you're experiencing problems accessing the site with one type of device, it could be worth trying a different device as the problem might only be present in one of the three versions.