The most common reason that people have this troubles is related to capitalization. All files are case sensitive. For instance, Images and images are not the same directories.
Linux File system is case sensitive, that is UppEr case and LowEr case matters. If you upload a file "Index.html" and you refer it like "index.html" you will get error as both are different files in Linux.
You need to uploaded a file with name "index.html". This file is used as your First Page or Home Page, that is when a visitor enter your web site, he sees content of the file "index.html". You can link other web pages in your web site from this "index.html" file. Make sure file name is in LOWER CaSe.