Cannot Access Administrator Site After Upgrade & Going Live
The problems with accessing Joomla administrator area might occur after upgrading Joomla from version 1.5 to 2.5 when Joomla component “jUpgrade” is in use. More specifically, it should show blank screen after attempting to login to administrator page. However, if following the guide how to migrate Joomla from 1.5 to 2.5 in most cases it should upgrade without any problems.
If you are facing with blank page issue while trying to login to administrator page, it might mean that there was an error during upgrade process. Since you do not have an idea what is causing that blank page, the first thing you might try is to enable PHP error reporting. This is something that you could ask for hosting service provider or if possible enable it by yourself by putting the following line in .htaccess file:
php_value display_erorrs On
This will turn on PHP errors and if there will be any of them, they will be displayed in website directly. Now you can check if any error is appearing while trying to login to Joomla administrator panel. If it is showing specific error, you might succeeded in troubleshooting this issue. However, if there are no errors, displaying PHP errors will not help us.
Another solution would be to check whether there is everything ok with Joomla configuration and .htaccess files. You should notice that when upgrading Joomla with jUpgrade, it is automatically creating separate sub-folder called “jupgrade” in the root directory of Joomla installation. There you can find a fresh installation of Joomla 2.5 version. Since you need to copy files located in this sub-folder to the root directory of Joomla installation and overwrite all the files, you can check if the contents of configuration file and .htaccess matches in both directories.
If you are still facing with blank page problem, probably there is not much you can do. Therefore, you could restore Joomla back to the 1.5 version if we have created a backup before doing any changes. At least you could have website working with older Joomla version rather than struggling with solving blank page issue in the newer version.