Hi Abhishek,
Was this user deleted from AD and then recently added back? If so, you have broken the link between the SID for the AD user and the AD GUID for the CRM user record in the database.
Microsoft best practice is to disable the user in CRM and AD so that this issue doesn't occur.
Microsoft can provide you a script that will delete the CRM user from the SQL database. You can then recreate that user again in CRM.
It would be possible to re-enable the user record from the database, and then re-associate the CRM user record with the new AD user from the CRM UI. But direct manipulation of the database is not supported by Microsoft, and is not something that I
would recommend doing. The best route is to contact Microsoft to fix the user.
Jason Peterson