Did you change your connection strings in your web.config?

When you copied your code base, did you copy all the files?

Is there any custom .NET code or .dll's with your install?

When you navigate to your site instead of loading the default page, try going to http://www.yourdomain.com/CMSSiteManager/Default.aspx, typically you can log in without issue. You might also check the server event logs. You could also query the Kentico event log directly and see if there are any errors/issues being reported you can't see

select * 
from CMS_EventLog
order by EventTime desc