Latest update destroyed my project

I would just revert back to the version of Premiere that was working. You can use Creative Cloud to install previous versions under "find additional apps" and select "previous versions" filter in the menu.

Well, as a learner, shall we say this is a learning experience?

A long-standing "best practice" in video post is NEVER update a project's software mid-project ... unless 1) you have to for some reason or 2) you've tested the project in the new version and everything you need works.

At this point, re-install the previous version and continue your project to completion in that.

Open your CC Desktop app, go to the "Find Additional Apps" bar, and if "Previous versions" isn't in blue on the right, click it and select that from the drop-down box.

Scroll down that to Premiere Pro, and click "Install"

That list allows you to choose a "build series" ... and installs the first of that particular build. 2015 is a 9.x build, and confusingly, 2015.3 AND 2015.4 are 10.x builds. Choose the one you were working in and install. Continue your project in that ...

Neil