Cannot access SQL Server - SQL Error 18456

Question

SQL Server Login is failing for one user where as others can access.

Login is Windows Authenticated (Part of AD group user and admin access)

I can see login in list of xp_logininfo members for that particular group in SQL Server Security

Mixed Mode is enabled (Both SQL & Windows Login)

But still getting

Login failed for user 'Domain\Username'. Reason: Could not find a login matching the name provided. [CLIENT: ]

Answer 1

Cannot access SQL Server - SQL Error 18456

You need to figure out what user does have rights to SQL server. Usually this is the user that you logged in with when you installed SQL Server or the user that is the default administrator on the computer.

I am assuming you have windows server 2012 and above.

I am write down some tricks to short out the Microsoft SQL Server , Error : 18456.

1. Launch Server Manager – Start – Click Server Manager . In the upper right corner, click Tools, then Computer Management. Expand Local Users and Groups (Under Computer Management-System Tools) then click Users

Notice under Description… There is a Built-in account for
administering… This is very likely an account that has access to SQL
server. Try logging onto windows with that account that is Built-in
account for administering then we can grant rights to the user you
want to use to login to SQL Server.

Now that you are logged into Windows with an account that has access
to connect to SQL Server, Let’s go in and grant rights to the user
that you want to be able to use to access SQL Server.

2. Launch SQL Server Management Studio again and you should be able to Connect. Expand your ServerName, then Expand Security, then Logins. Click Logins

Right-Click Logins Select New Login…

Click the Search Button

Type in the Windows User Name you would like to add as an SQL
Administrator then click Check Names (or you can click advanced and
select from a list) 

Click Server Roles in the left pane; then turn on the sysadmin
checkbox, then click OK.

You can then confirm you have sa rights by double-clicking the user
name in the left Object Explorer and clicking on Securables.

Now you can log out of Windows, login as the user you just granted
rights to and all should work fine.

For your further ref : Here