Having upgraded to kernel 5.3.18-150300.59.49-default, the screen stays blank
once the system has booted.  The problem seemed similar to bug 1195142 - i.e. a
bug in the radeon driver.  I solved it in the way suggested there.

Booted with the ...-46-default kernel, then as root:

>mkdir 5.3.18-150300.59.49-default/updates
>cp /lib/modules/5.3.18-*.46-default/kernel/drivers/gpu/drm/radeon/radeon.ko*  /lib/modules/5.3.18-*.49-default/updates/
>depmod 5.3.18-150300.59.49-default
>mkinitrd

Restarted -> system started as normal.