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
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