SQL SERVER ERROR 18456

MSSQL Server Error 18456

First of all, you just can't connect to your SQLServer if this error occurred. It's highly likely that you've just installed SQLServer in your local machine if you encounter this error. This error happens because you haven't turned on SQLServer Authentication mode yet. See the details of this error at https://docs.microsoft.com/en-us/sql/relational-databases/errors-events/mssqlserver-18456-database-engine-error

Steps

1. Right click on a database server and click properties.
2. And move to security tab
3. Server Authentication, 
4. Select SQLServer and Windows Authentication mode.

Click ok and restart SQL Services. Don't just close SSMS. It shall work fine.