Situations when a user cannot log in suddenly?

Hi,
I am currently using the login wizard for my form authentication.
However, I'm now having the issue of a user being able to log in. But suddenly the user cannot log in again, with the standard error message that the username or password is wrong.
From what I researched, this can happen if the user is locked out, after providing invalid password for a number of times within a period of time.
However, my user did not seem to provide any invalid password.
Therefore I am wondering is there any other situations where a user cannot log in suddenly.
My login restricts access of same user at a same time.
Under the LoggingIn event, I use the membership to check if the user has already been logged in. If yes, it will bring the user to a page, telling him or her that the user has been logged in.
 
Protected Sub Login1_LoggingIn(ByVal sender As Object, ByVal e As System.Web.UI.WebControls.LoginCancelEventArgs ...