There is a known issue with side by side installations of Revit 2024 and 2025 where uninstalling one will results in the removal of the materials files necessary to populate the Graphics tab of the Materials browser. At this time, the best solution is to reinstall the version of Revit that was uninstalled, which will restore the missing materials files. Another solution would be to contact Autodesk support, who should be able to provide a zip file from which the missing materials files can be restored.
If your attempts to reinstall Revit 2025 end with a message that this release is already installed, I would recommend searching for and deleting the following registry keys:
HKEY_LOCAL_MACHINE\SOFTWARE\Autodesk\UPI2\{FD1D3188-485A-40A9-8D09-0E0817E8299B}
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Products\0A4B643700520150000007C5D0680240
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\{686CE2A3-7C33-3AD5-806A-75A6E648117F}
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\{7346B4A0-2500-0510-0000-705C0D862004}
Anthony MichniewiczPrincipal QA Analyst