Help help help!

Okay, I have this nifty CMS on my site, but I've screwed it up somehow. I can add and delete stories, but when I try to edit them I get the message "You don't have permission to access /cms/index.php on this server." Not sure how this can be true, because I still have adding/deleting access, but apparently it is. Does anyone have an idea about how a CMS-ignorant person might go about fixing the problem?