The email doesn't arrive

Hi,

Do you see the email in the Customers>Emails history menu ?
Is the "order notification" email still enabled under the System>Emails menu ?
These are the two first things I would check.
If the email is disabled, then you need to enable it.
If the email is in the email history, then HikaShop sent the email to the email server of the hosting. The issue with it will be logged in the log file of the email server and there is no way to know why it was not delivered from your website itself. Your hosting support should be able to look at the log and tell you.
If the email is not in the email history, then a plugin is cancelling the sending of the email for some reason. For example, AcyMailing has / had a plugin to override the emails of Joomla and HikaShop. What it would do is actually cancel the sending to send its own version:
docs.acymailing.com/main-pages/email-overrides
It seems however that the support for the HikaShop emails has been dropped recently.