Hi,
Thanks for the access.
The issue is that you're on HikaShop 5.0.0 on your website, not 6.4.1 as you stated in your message.
If you update Joomla, you also need to update the extensions, otherwise, they will break at some point because Joomla changes things. I can see javascript errors on your product page with variants. These come from changes Joomla made in how it handles AJAX requests. These changes lead to HikaShop's javascript breaking. We've had to make changes following this so that it works properly again.
Update your HikaShop to the latest version and it will work fine again.