@Raghu
What is the type of authentication method you are using? What is the URL you are using?
To troubleshoot this issue, check things below:
1.Try to use farm administrator account to open the root site collection.
2.Go to IIS -> Application pools -> Select the application pool -> Advanced settings -> Make sure that use farm administrator account as identity.
3.Go to IIS -> Sites -> Select the site -> Double click "Authentication" -> Windows Authentication -> Advanced Settings -> Check the box for "Enable Kernel-mode authentication".
4.According to this article, disable LoopbackCheck.
5.Check the detailed information about this issue in the ULS logs.
Reference:
Troubleshooting - 401 Unauthorized Errors
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