Troubleshooting Login failed Error 18456

Input : select * from sys.sysmessages where error = 18456

Output: Login failed for user ‘%.*ls’.%.*ls%.*ls

This is one of the infamous error message (and number) that most of the DBAs and developers have come across while working on SQL server. This message simply denotes that the client call was able to reach the SQL server and then an ACCESS was denied to the particular login for a reason. To figure out the exact reason, this error number 18456 with its STATE number is logged into the SQL server error log file, if SQL server was allowed or configured to capture the failed logins.