The most common reason for a web-site to be down can be separated on few major categories.
The first category is a web-site down with an error massage. In other words there is certain reason for the web-site to be down and this reason is provided with an error massage. Depending on the error massage the reason can be from connectivity issues to errors in the code files. Usually an error code is provided or a Warning/Error massage related to the code is displayed.
The second catagory is a web-site down with an blank page. The most common reason is that the content in the index.php/index.html is empty line. In other words the file is existing but it is an empty document.
The third category is when you see a list of the files located in the public_html folder without any content is displayed. The most common reason for that issues is the fact that there is an missing DirectoryIndex directive in the .htaccess file or simply missing index.html/index.php file.