1.5.2 Comment notification not working

‘blah,blah,blah’ – that could cover a multitude of events, of which only one could have broken this.

Check that all your core files are present and correct.
Go over every change you made and if possible reverse it to try and establish what broke it.

The only change I made to my core files was to move them from a subdirectory to the index. All the other changes were to the custom template only. Do you have any more-specific ideas? As I said, I searched the boards for this problem, which seems common, but the fixes apparently don’t apply to 1.5.2, as I do not have the line they all say to change in my functions.php file.

The ‘common’ part of posts here about comments not being sent is usually that there is a hosting problem. Many people never get any emails sent whereas you did have them and then they stopped.
There is a chance of course that while you were making the site how you wanted, that your webhost also made some backend changes and the email function has been affected. Email them to see ?

You could try installing a standalone contact form that does not rely on WP at all – if that works, WP is broken. If it does not work, it’s the host.