Single pages not “themed” after upgrade

I’ve been searching through the forums, but nobody seems to be having the same problem, so I’d be glad if someone could help…

I just upgraded from 1.2.1 to 1.5.2 and would like to use a theme I’ve been using that I created myself. I’ve followed the instructions to the letter (I think), and the start page is looking fine, i.e. just as it used to. But the single post pages aren’t themed at all, they are totally screwed up.

Is this normal? Do I have to build a template for the single pages from scratch? (I guess I could use my index.php for this, as I want my single pages to look mostly the same as the main index.) If so, which templates do I need to edit? header.php, single.php? Something else?

Thanks in advance…