Black screen with iMac 14,2 system definition

I hope some one can shed light on this, excuse the pun, I have experienced this issue only since upgrading to 10.12.2 and on two machines.

This is on the machine in my signature. This has worked flawlessly for some considerable time but has today suffered this issue. I have also seen this on a test machine but I rebuilt that one.

I went to wake the system as normal this morning but after wake I have a black screen with a mouse pointer that works. Notifications flash up on the screen so am...

Black screen with iMac 14,2 system definition