“index.php” not appearing automatically

I’ve tried searching but am probably not using the right search terms.

I’ve created three blogs with WordPress…

With all three, you simply typed “mywebsite.com/blog” in your browser window and it would appear.

Now ony one works this way — the other two simply display what is in the folder. You must type in “mywebsite.com/blog/index.php” to get the blogs to appear. And if you click a Category, you’re right back in the raw folder view.

I have upgraded all the sites to 2.0 and the problem site exhibits this same behaviour under both 1.5.1 and now 2.0. I did not change anything before this behaviour started appearing.

Same behaviour in both Firefox and Internet Explorer.

Any help would be gratefully received.

Thank you.

Don