how to enable sa login in sql server

step: 1

   (i)right click on the server and select properties.
 
   (ii)select security

   (iii)enable the sql server and windows authentication mode

   (iii)restart the sql server

step: 2

    (i)go to service.msc in find sql server (mssqlserver)

    (ii)click on restart

step: 3

    (i)in sql server go to security/login/sa click here, set login name and password

    (ii)in login property status select enabled

    or:

    sql server windows athentication mode open connection using query enable

    go
    alter login sa with default_database=master
    go
    use master
    go
    alter login sa with password='123456'
    go

 *note : you can already set username and password change or alter above query..