Currently your SQL Server is configured for Windows authentication thats why you are not able to login as SA.
(a) login to sql server using windows administrator privillaged user
(b) Change to Mixed mode. SQL Server --> Right click --> Select properties --> In server properties window ---> select security --> Select SQL Server & WIndows authentication --> apply
(c) enable the SA login. Right click on SA --> Property --> enable login,
(d) restart the service
Madhu