I had a very simple site with the following linking pattern:

www.main.com - showed index.php and linked to subpages like
www.main.com/subpage - has its own index.php
www.main.com/subpage1 - has its own index.php
www.main.com/subpage2 - has its own index.php
all the pages have the same banner linking them between each other.

Everything was working fine a few days ago and now, suddenly every link shows the first index.php. I just don't get how www.main.com/subpage can show the index file of www.main.com when it has its own

I haven't changed anything on the site. Some kind of server-side change?
But what could cause that index-file matter?

Many thanks in advance!