Mmm.
I don't think that it could be related to the email settings since you are receiving the other emails.
As I already said, there are only two possibilities as far as I can see:
It could be that you unpublished the order admin notification email via the menu System->Emails or that you configured it in a way that doesn't work.
If most of the text of your email is missing when you edit it, there is definitely something wrong with your email as it is not like that by default and that probably causes the issue.
Now I don't see why restoring that file wouldn't make it work again.
A reinstall might indeed be a good idea and you should remove the media/com_hikashop folder and the hikashop_config table in the database before. That way, it will reset all your settings of HikaShop and the email related files.