Why all Matlab windows blank and unresponsive?

Today I was coding in Matlab then went to lunch. When I returned, all the Matlab windows (editor, command window, 2 figure windows containing images of size 821x821) are entirely blank except for the heading such as 'Figure 2'. No calculations or simulations were running when I left, and I am running R2019a on Windows 10. Any ideas what could have caused this, which I have never seen before?

[KAE]

When I restarted I got the following message which seems to explain it, but I will not delete my question in case it helps someone else,

Warning: MATLAB previously crashed due to a low-level graphics error. To prevent another crash in
this session, MATLAB is using software OpenGL instead of using your graphics hardware. To save
this setting for future sessions, use the opengl('save', 'software') command.

In my startup command I have the following line,

opengl hardware % Make lines more smooth

So I will change it to the following and hope that does the trick,

opengl('save', 'software')