Yesterday I have installed SQL Server 2008 on my windows PC, while installing it i chose the "Enforce Password Expiration" option. After completing the installation process i have verified my SA password also user account Password then it works fine. But, Now when I am trying to login with it, It gives me login failed error 18456. why? how can i fix this issue?
Is there another account that has sysadmin membership? If so, you could use that to reset the sa account password. As always, make sure that you are using the correct password, including checking for the CapsLock key. If those don't help, I don't know of a way to restore the sa account outside of reinstalling the software and attaching any user databases.
If @stephen_hendricks suggestions did not help, you can try using the dedicated administrator connection as described here.