Hi,
Hard to say. If it's sudden and you didn't change anything on the website, it's likely coming from the email server of your hosting.
First, check that you still get the emails in the "Emails history" menu of HikaShop. If you do, do a screenshot of one of the emails details and contact your hosting company support with the screenshot explaining that the emails are not sent anymore from your website even though your website says that it sent them.
