Uhmmm, Thats wierd in my case it always pick it up automatically anyway, if you pay an external hosting you should contact their support, in the other hand if you are managing your own server then you have to find something to make a default content, for example in IIS its called Enable Default Content Page.
try it by writing entire code of index.html into default.aspx
You need to set index.htm as the default document in IIS.
Check this link.
If you are using a web hosting company to deploy you site over internet, you can ask them to set index.html as default document for your site.
thanx for the suggestion