Getting Error Message for Exception System.ComponentModel.Win32Exception (0x80004005): Access is denied while accessing sub site. Able to access root site.
You can troubleshoot your issue as following:
Go to subsite Site Settings->site permissions->Check permissions, check if the current user has permission to access this subsite.
What’s the browser you used to access the site? Try to use other browser and check if the issue exists.
If you encounter this issue in IE, go to Tools->Internet options->Security tab->Custom level->scroll down to User Authentication section, select “Automatic logon with current user name and password” or “Automatic logon only in Intranet zone” and check again.
If the issue still exists, you can check the ULS log to get more error message about your issue.