Hi,
Edit the file administrator/components/com_hikashop/pluginCompat.php via FTP and replace its content by this :
<?php class hikashopJoomlaPlugin extends JPlugin{} ?>
That should allow you to access your website normally.
The issue is linked to the changes we made for the support of Joomla 5, which required code which can't be compatible with PHP5 to be loaded at some point. So, following your report, I've now made a patch so that this bit of code is only parsed when using Joomla 4 or greater. Since Joomla 4 can't work on PHP5, it will fix the problem.
So once you get back access to your website with the change mentioned above, you can download the install package of HikaShop 5 again on our website and install it on yours and it will work without that error.