I had to hard boot my MacBook off last night after it got stuck in a Keynote presentation.
On the restart, I opened Safari / Chrome and would visit a webpage and about a second after rendering the window would go white (clear on Chrome).
I did a proper restart, and it came back up to the grey screen before you can login. (Mouse active. Functions volume, brightness etc active.)
It was stuck there all night and occasionally it would flicker. I rebooted this morning into safe mode which worked fine. Rebooted and came to this screen again.
I'm not really sure what the next steps should be. I've never really had to troubleshoot my Mac.