A blank screen typically indicates that the browser has poor/broken connectivity with the your institution's server, and the webpage content cannot load into the browser. 

In most cases, you should be able to leave your exam, shut down LockDown Browser, and restore your internet connection. We recommend restarting your computer and moving as close to the router as possible. It may also help to restart the router.

If a freeze has occurred, and you're unable to exit, please reference this article.

Then, start up LockDown Browser again and return to the exam (assuming your institution's Learning Management system permits the return.) In most cases, your school's learning system will have auto-saved your answers, so you should be able to pick up right where you left off.
Keep in mind that if use of a webcam is required for the exam, you'll need to again navigate the pre-exam webcam check.

A blank screen can also occur if you have multiple monitors hooked up. This is not supported. Disconnect the additional monitors and open LockDown Browser up in the main monitor.

Finally, a blank screen can happen if the device you're using doesn't have sufficient memory (RAM). You may need to use a device with more memory.