Hi,
Could you see the C drive in Device Management\disk management?
You can check the Group Policy about hide the C Drive.
Group Policy
User Configuration > Administrative Templates > Windows Components > File Explorer
Set "Hide these specified drives in My Computer" to restrict the C Drive
My Drive does not appear in Windows Explorer view
https://support.microsoft.com/en-us/topic/my-drive-does-not-appear-in-windows-explorer-view-80b83adf-ab7f-2c11-c9df-532c8b6fc438
Hope this helps and please help to accept as Answer if the response is useful.
Best Regards,
Carl