I have several sites running on 4.3.5.  They have all been working fine then suddenly two of them no longer respond to logins.  None of the site logins work.  You enter them click login and it just clears the boxes.  I tried using the following script, which has worked in the past to reset a lost host password, to no avail:

Declare @UserName NVarChar(255)
Declare @NewPassword NVarChar(255)
Declare @PasswordSalt NVarChar(128)
Declare @Application NVarChar(255)

-- Enter the user name and new password between ''
-- do not leave any spaces unless intended to do so.
-- Edit only between single quote characters
Set @UserName = 'host' -- This default DNN host user
Set @NewPassword = 'newpassword' --New password
-- Do modify any code below this line

Set @Application = (SELECT [ApplicationID] FROM aspnet_Users WHERE UserName=@UserName)
Set @PasswordSalt = (SELECT PasswordSalt FROM aspnet_Membership WHERE UserID IN (SELECT UserID FROM aspnet_Users WHERE UserName=@UserName))

Exec dbo.aspnet_Membership_ResetPassword @Application, @UserName, @NewPassword, 10, 10, @PasswordSalt, -5

I also tried to use the following procedure but found out that I cannot register a new user either.

As there are frequent requests from users, who lost the password of the host account (and you cannot request the host password by email), the following solution will help for all DNN 3 and 4 installation. Please note, that your need direct access to the database to suceed with the issue:

-If "register" is not displayed for the portal, go to table "PortalSettings" in your database and enter value "2" into column "Registration".
-Create a new user account by registration (this time, please remember the password you enter!)
-Go to database, enter table "ASPNet_Membership"
-Go to new user account (usually the last one) and copy the encrypted values of columns "Password" and "PasswordSalt" into the same columns of user account "host" (usually th first entry in this table) 
-Login as user "host" using the new password and delete the reshly created other user.

Although both of the above are great tools ... neither works for my situation.  Does anyone have any idea what would cause both login and regsitration to quite working?  Any help would be greatly appreciated.