The user attempted to login with credentials that cannot be validated. Possible causes are:
The login may be a SQL Server login but the server only accepts Windows Authentication.
You are trying to connect using SQL Server Authentication but the login used does not exist on SQL Server.
The login may use Windows Authentication but the login is an unrecognized Windows principal. An unrecognized Windows principal means that the login cannot be verified by Windows. This could be because the Windows login is from an untrusted domain.
Similar problems can cause the less-specific error 18456.
If you are trying to connect using SQL Server Authentication, verify that SQL Server is configured in Mixed Authentication Mode.
If you are trying to connect using SQL Server Authentication, verify that the SQL Server login exists.
If you are trying to connect using Windows Authentication, verify that you are properly logged into the correct domain.
If the user listed in the message is NULL, there was a problem with the Windows Authentication passed to SQL Server. The problem could be due to many factors, including incorrect name resolution, an out of sync domain controller, no available domain controllers, SSPI problems, or policies or permissions settings that prohibit connections.
If the error occurs when one computer is attempting to delegate Windows-authenticated permissions to another server, this error may indicate that the Windows delegation, often called a double-hop, is not properly configured within the domain. A common example of a double-hop is a linked server query where the client connects to an instance of SQL Server, and that instance tries to delegate the Windows permissions to an instance of SQL Server on another computer.
For more information about Windows Authentication, see Microsoft Knowledge Base article 269587.
If the user listed in the message is NULL:
Additional information gathering steps that can help to narrow down the problem: