Hi,
I am also having the same issue - I followed the advice above (new checkout page and inserting the shortcode into the plain page text editor and not cornerstone) but no luck. The page loaded without the checkout shortcode but as soon as i added the shortcode it stopped loading.
When cornerstone is deactivated the checkout loads fine (however obviously all the other pages look awful!) the following error appears in the error log:
[04-Feb-2019 21:13:18 UTC] PHP Fatal error: Allowed memory size of 805306368 bytes exhausted (tried to allocate 32768 bytes) in /home/SITENAME/public_html/wp-content/plugins/woocommerce/templates/checkout/form-checkout.php on line 1
[04-Feb-2019 21:13:18 UTC] PHP Fatal error: Allowed memory size of 805306368 bytes exhausted (tried to allocate 32768 bytes) in /home/SITENAME/public_html/wp-includes/functions.php on line 3778
[04-Feb-2019 21:13:18 UTC] PHP Fatal error: Allowed memory size of 805306368 bytes exhausted (tried to allocate 32768 bytes) in Unknown on line 0
It’s weird as it all worked fine but then it randomly broke!
Any help would be greatly appreciated!
Thanks,
Dan