In your store's Admin area, go to the System > Logs > Error Log
A full log of every error message your store experiences. Review the error messages in case of any problems or click Clear Log button to reset error log.
Also You can disable error loging in System > Setting > System Tab. Find Log Errors setting and set it to OFF
In case you can not access admin you can locate error log directly in the file /system/logs/error.txt