A blank screen indicates that the browser has lost connectivity with the your institution's server, and the webpage 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.
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 LMS server 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.