If it's an apache server the httpd.conf file has an option called DirectoryIndex where you say the names of the index file.
If you don't use apache then whatever you use is probably similar.
# DirectoryIndex: sets the file that Apache will serve if a directory
# is requested.
DirectoryIndex index.html index.html.var index.php
The above would show index.html first if it exists, next index.html.var, then index.php. If nothing exists it reverts to showing a list of files in the directory, if that option is turned on.
If you host your own server you will need to set this up, or if you pay for hosting you will need to ask our host what the filename is (probably index.html or index.php).