Error Number 18456: Login Failed For User

When an attempt to connect is rejected because of an authentication failure that involves a bad password or user name, a message similar to the following is returned to the client: "Login failed for user ''. (Microsoft SQL Server, Error: 18456)".
The following message might also be returned:

Msg 18456, Level 14, State 1, Server , Line 1"
"Login failed for user ''."

To troubleshoot this failure, look at the error state that maps to an authentication failure condition. Compare the error state to the table list below to determine the reason for the login failure.