Have for a while experience issues with one website based on WP going up and down and not being available.
Checked “WooCommerce logs in dashboard”

At lot of fatal errors at the exact timestamp as the site became unavailable. A quick search revealed that I’m hitting the php memory ceiling of default 64M.
Solution: Upp the php memory to 256M
Located the wp-config.php file and added
define( 'WP_MEMORY_LIMIT', '256M' );
Just before the line
That’s all, stop editing! Happy blogging.
Restarted Apache
/etc/init.d/apache2 restart
Entered su password and done!