How to fix Microsoft SQL Server Error: 18456, cannot log-in.

I've encountered this error many times when I tried to log in to my SQL Server prior to assigning administrator privileges to the user. This error occur because the user you use to connect to the server using windows authentication doesn't have a permission to log-in or your username and password are incorrect.

The Error look like this.

Event ID :18456 
Connect to Database Engine
Cannot connect to "SQLServerName"
Additional Information: Login failed for "your user name".(Microsoft SQL Server, Error 18456)

But the common cause of this is the username you use is not a member of administrators group.
This is how to solve it.

1. Go to control panel > computer management
2. On expand to Local user and Groups > Users
3. right click on the user you want to use to login to SQL Server then select properties
4. Then on Member of tab, click add then type administrators

This applies to MS SQL Server Express 2014, MS SQL Server 2013, MS SQL Server 2012
Try again to log to your MSSQL using the user you grant permission as administrator.