First Web Page blank

[#1]

I tried to open my web site this morning using Chrome and got a blank page.  The same thing happened with IE10.  Looking at the site with an ftp client the index.html file size was 0 bytes (normally 450)!  Something had screwed it up so I replaced it with a copy of the original.  This made no difference.  Renaming it to index.htm made no difference either.
If I open the site in a browser via http://www.<username>.plus.com/ the screen is blank.  However  http://www.<username>.plus.com/index.html brings up the first page.  ftp is working normally.
Any ideas?

[#2]

Is there a .htaccess file in your root?
Do you have antivirus software on your PC?
Sounds suspicious.
Gabe

[#3]

Solved it  by clearing the browser caches.  However, I don't know why the index.html file had lost its contents.  On investigation all other .html files on the site were also 0 bytes!  I've made them all read only now in case someone manages to hack in.
There isn't an .htaccess file in the root and I do have antivirus software.