First, props for thinking to make a backup. Very smart. But do consider using source control (see below).
To fix this immediate problem, try a reimport-all. (right-click on project window).
Another thing to try is to create a new HDRP project (named differently), then copy over the Assets folder from your current project to this new project.
My standard source control plug below:
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.
As far as configuring Unity to play nice with git, keep this in mind:
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:
Share/Sharing source code between projects:
Setting up the right .gitignore file:
Generally setting Unity up (includes above .gitignore concepts):