Blank cart page when upon clicking Order Now or Proceed to Checkout
Blank cart page when upon clicking Edit Cart or Continue Shopping
UI Macro servicecatalog_cart_template was customised
Revert the UI macro servicecatalog_cart_template to out-of-box (OOB) version and test again
If the above workaround does not work, you can check the 'Node Log File Browser' and filter in the Start Time and End Time of when you click the button.
Check for further errors on the logs.
You can also check your web developer console to check for further errors.