The problem is that the main ecwid script is not included on the page. At a glance I can think of two possible reasons for it:
1) There is some kind of javascript optimisation plugin in your site that has cut the ecwid script inclusion. Do you have such plugins?
Most of these plugins have a feature of adding scripts to a special list that is not affected by those procedures. So, If that is the case, then, please, add Ecwid script to that plugin exceptions. Adding “app.ecwid.com/script.js” could be enough, however, if the full form is required, then it will be “//app.ecwid.com/script.js?<STORE ID>”, where <STORE ID> is your Ecwid Store ID.
2) The store page content does not have [ecwid_script] shortcode now. It could be occasionally deleted, for example. Could you, please, check the store page content?
If that is the case, then, please, put [ecwid_script] shortcode before other ecwid shortcodes, it should solve the issue.