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Users are most commonly locked out and cannot be validated by the Membership.ValidateUser(string, string) method when the Membership.MaxInvalidPasswordAttempts is reached within the Membership.PasswordAttemptWindow.
Users can also be locked out if you use the MembershipUser.GetPassword(string) or MembershipUser.ResetPassword(string) overload that accepts a password answer and the number of bad answers entered by the user reaches the Membership.MaxInvalidPasswordAttemptswithin the Membership.PasswordAttemptWindow.
To set this property to false and let the user try to log in again, you can use the MembershipUser.UnlockUser method.