First, check if the battery's charge didn't run out. If it did, plug the charger into the laptop.
If the fan was running loud and the laptop was hot, then too many programs were running. On start-up, try running less programs and make sure there's good airflow to the fan.
Otherwise, the fan could be worn out or broken. Go to the [[Asus F6A-X2 Troubleshooting#Section_My_laptop_shuts_off_by_itself|troubleshooting page]] for more details.