if you have access to your database, e.g. using SQL Server Management Studio, open Portals table and overwrite Value in column LoginPage with "Null" (strg-0 or copy from other field).

Hello, I am having the exact same problem in 2013... 

I upgraded my DNN site from 4.8.x to 6.2.8 and no errors at all displayed.... however there is NO LOGIN PAGE

I've opened the SQL Management Studio opened Portals and only find the following table columns.

PortalID, ExpiryDate,UserRegistration, BannerAdvertising, AdministratorId, Currency, 
HostFee,HostSpace, AdministratorRoleID, 
RegisteredRoleId, GUID ,PaymentProcessor ,ProcessorUserId ,ProcessorPassword ,SiteLogHistory 
,DefaultLanguage,TimezoneOffset, HomeDirectory, PageQuota ,UserQuota,CreatedByUserID,
CreatedOnDate, LastModifiedBy,LastModifiedOn, PortalGroupID

Do not see a LoginPage column in my DNN 6.2.8 upgrade...

By the way, everytime i try to open or access the login page i get
"A critical error has occurred. An unexpected error has occurred"

This is strange because i did not get an error during upgrade.