showing 404 instead of the index file – rest is working fine

I got suddenly problems with my site. The index file doesn′t work anymore. It′s showing the 404 page instead. I have tride to upload an old backup (that i know worked) with out success. All other files seem to work properly, also tha admin files.
What do I do? I have spend most of the day trying to solve this without any luck. It′s frustrating.

Xtra: When changing themes you get a nie preview of the theme, before activating the selected theme. I went back and forth between some themes , and in this preview the wanted them is displayed correct – not showing the 404 file. How can this be possible?