"C:\ is not accessible. Access is denied" error message
In NTFS volume, only the owner of an object (file, folder, ...) that can change permissions of that object. You need to take ownership of your drive C: somehow, after that only you can grant permissions to users.
Try to enter safe mode, login as "Administrator" (enable that account first if it is currently disabled). Then right click C:\ drive, click Properties. Navigate to Security Tab, then click Advanced. Go to Owner tab, then change the ownership to "Administrator" first. Then, grant permissions like these to these users [CMIIW]:
Authenticated Users: Read, execute, write, modify,
Administrators: Full Control
TrustedInstaller: Full Control " in windows 10 trusted installer is NT Service\TrustedInstaller "
SYSTEM: Full Control
Users: Read, execute
Finally, change the owner once more to "TrustedInstaller".