Maybe the website is down because of an ISP- or hosting-related issue. Try again in 10-30 minutes to check if it's back online. If it's still offline, report the issue to the hosting provider which will resolve it quickly.
When you type an address into your web browser, your computer uses DNS (Domain Name System) to look up and translate that address into a computer-friendly IP address. In other words, the DNS has the role of a librarian, who you can ask for a book. Sometimes, the servers your computer uses to look up that information can go down, thus preventing you from loading the desired web site.