If you are trying to login to a SQL server and you are receiving just the generic 18456 error, here is how to figure out what is going on.

1. Open up the Microsoft SQL Server Management Studio tool.
2. Login with an account that has administrative privileges.
3. Click on Security and make sure under "Login Auditing" you have at least Failed logins only checked or higher.
4. Click OK.
5. Try to login with your account on another server to trigger the 18456 Error
6. Go back to the SQL Server and open up Event Viewer (Start->Administrative Tools->Event Viewer)
7. Click on Windows Logs->Application
8. You should see Event ID 18456 inside of your logs (It won't be Critical or Error level, just Information).
9. Double click on your event, and it should give you some information on why the account is unable to login.

For example, in my case, I was attempting to use SQL authentication on the server, and the log told me the server was only setup for Windows Authentication (sure enough, it was! :P).

Hope this helps!