I have SketchUp 2021 and since I updated to this version the default materials have disappeared. I only see materials that I had placed in old files.
When did you update? Did you install it correctly? That requires right clicking on the downloaded installer file and selecting Run as administrator.
Try repairing the installation. Close SketchUp, go to your Downloads folder and find the installer. Right click on it, choose Run as administraotr and when presented with the option, choose Repair. After the repair is complete, reboot your computer.