If the exam is frozen hold down the power button and the home button at the same time. This will do a hard restart of the device. Since you are still in guided access this will boot the device immediately into the ST-M app.
After the reboot and re-entering into the app you will see that you can enter in a Resume code. This code will allow the Exam Taker to enter back into the exam. This code can only be provided by an individual that has access to the Administrator Portal.