Access Denied on the site collection after migrating form SharePoint 2010 to SharePoint 2013.

Problem:

This problem happens if you are using the detach and attached database approach for the migrating SharePoint 2010 to SharePoint 2013 because the old site collection administrator user is already stored in the content database  

Solution:

1-      If you connected the same domain you will not see this problem otherwise

2-      Open the Central Administration and under the application management

3-      Select "Change Site Collection Administrator" then you will find the old site collection administrator   

4-      Then change the old site collection administrator with the new one