By default, Magento does not display messages about errors and exceptions; so if something bad happens, you don't even know what is the error or exception.

To turn on the error logging, you can navigate to System > Configuration > Developer > Log Settings from the magento admin panel  and set the «Enabled» option to «Yes»

The error logs are stored on this "/var/log"  magento path, so set  full access permissions (777) to these folder.