It could be a few things:
Is your domain completely propagated? It may take up to 72 hours for this to complete. Propagation is the process of having all the world's servers recognize the changes you made to your domain. For example, when you change your nameservers to Avangard Hosting so your site can be seen using the Avangard Hosting' servers, it will take up to 72 hours for all the world's servers to recognize this change.
Did you upload your site to the correct folder? All files need to be uploaded to the public_html folder
Do you have your first page named index.htm (.html, .php, .asp, .jsp...)? Please remember that Linux servers are case sensitive, so be sure to use "index" not "Index".