This goes without saying, but just to check off a possible cause: Did you try gmail's 'all mail', 'spam', 'notifications', etc... labels? Gmail seems to find new and inventive ways of hiding automated emails in different folders.
If another email account works fine and Gmail doesn't then the problem is probably closer to your end, if neither work then it's probably closer to the HN end.
There are some awfully bizarre ways that emails can fail without the sender or receiver really having done anything wrong.