That seems really weird. I assume your HubSpot portal is still active and you can login, right?
If so, I suggest you uninstall the plugin and reinstall the latest version following the links and instructions at Install the HubSpot WordPress plugin
If you still have problems with the tracking code disappearing (maybe there is a plugin conflict or something) you may be able to work around it by hard embedding the script using a script insertion plugin or by editing the header file, if you know how.
An example of such a plugin is Insert Script In Headers And Footers but I have not tested it and if using it, or any other, you do so at your own risk.
I have manually inserted the HS tracking code in to my WP theme footer as instructed here and it appears in the page source. This however has not resolved the issue.
I have removed and reinstalled the plugin (twice) and the forms are still not appearing on the page, please see here under the heading "wedding car hire quote form". There are just blank white boxes. The embed codes are all correct as well.
When I view the page source it says the following in the header with no script in between:
<!-- DO NOT COPY THIS SNIPPET! Start of Page Analytics Tracking for HubSpot WordPress plugin v7.5.2 -->
<!-- DO NOT COPY THIS SNIPPET! End of Page Analytics Tracking for HubSpot WordPress plugin -->
I deactivated WP Super Cache yesterday thinking it would fix the problem, turns out it was Autoptimize causing the forms to not appear, must be one of the settings conflicting with HubSpot.
This is the setting causing the problem: "Also aggregate inline JS? - Let Autoptimize also extract JS from the HTML. Warning: this can make Autoptimize's cache size grow quickly, so only enable this if you know what you're doing."