I went in the advanced setting and disabled mage core. I have no more admin area. I do I restore mage core.
Run the following SQL query in your phpMyAdmin or mysql console:
UPDATE core_config_data SET value=0 WHERE path='advanced/modules_disable_output/Mage_Core';
The query above assuming your database table prefix is empty, otherwise append 'core_config_data' with your database table prefix, which is defined in 'app/etc/local.xml'.
After that Flush Magento cache by deleting all subfolders of var/cache/ directory, or run (ask your hosting to run) the following command in your SSH console:
find ./var/cache/ -type f -delete
Replace ./var/cache/ in the command above with full path to var/cache directory of your Magento installation.