SQL Server error 18456 (Login failures)
With a little help from SQL Server’s tracing tools you can easily get to the bottom of almost all login failures (18456 errors).
If the login failure is against SQL Server 2005 or above and the default trace is running just go to my SQL Server default trace post.
If you’d prefer to use extended events (as SQL Profiler is now deprecated), go to Capturing login failures via SQL Server Audit.