Why is index.php not showing up in the SMF forum?

Question

PHP

Good day, dear Khabrovites!
The situation is as follows - I set up a forum on a free smf engine. After (I think so) manipulating the theme settings, the index.php homepage is no longer displayed. But at the same time, when you add the script address to it (for example, index.php?action=login), you get where you requested, the page is displayed.
Please help someone with what they can and not get angry at the noob for stupid questions!
I'll be honest - I smoked Simple Machin's fakes and forums for a long time before getting to you, but no.