I’m sorry you’ve had this issue. I assume you’re not using source control? If you are using source control, just revert and clean and it will be back where it was on the previous version. But without source control you cannot reliably go back from an upgrade except with a full project backup.
As for the rendering pipeline stuff, you might be able to fix a LOT of stuff by removing some of the package crap that Unity tends to barf into new projects, often conflicting with stuff already in your project. Also try updating all the packages you are using to latest.
Please consider using source control in order to guard and protect your hard-earned work.
Personally I use git because it is free and there are tons of tutorials out there to help you set it up.
Here’s how I use git in one of my games, Jetpack Kurt:
Using fine-grained source control as you work to refine your engineering: