Connecting iPad 'crashed' the display - black screen but cursor responsive Difficult to describe exactly what happened, but best as I can...


Mac Pro 08 running El Capitan; all else up to date.  
Plugged iPad into a USB cable coming via the wired keyboard.  
Mac screen went entirely black. Mouse cursor was still active, but no keyboard input would be accepted - any command would give the 'oops, wrong key command' beep.  
As far as I could tell, all apps were still running correctly, behind the scenes, though the only real evidence was that the internet radio was still playing in Safari.
Unplugging the iPad made no difference.
Prime consideration - I didn't want to hard power-off the Mac, risk losing work &amp;/or [this has happened before] not being able to boot up until I'd fiddled with Disk Warrior/TechTool etc


Incidentally, the iPad shows as Not Charging. I haven't yet investigated whether this is because it's connected through the keyboard rather than directly.

I need to do further research to see what may have been the cause, but I'm going to post an answer myself as to how I got out of the black screen, in case it's useful to others...
Because I didn't want to hard power-off the Mac I had to do a bit of lateral thinking.  

I was quite sure that the machine hadn't actually hung, as the radio was still playing, but that perhaps something had upset the Finder, which I think was the frontmost app at the point the iPad was plugged in.

A moment of revelation... what about dialling in remotely?  

I went to my other Mac &amp; tried to get in via Screen Sharing.
Success... or not... I could share the screen, but it was still black with just a working cursor.  

What if I try to login as myself instead?

Joy!
Screen Sharing showed me the login screen.
I went to check the remote machine, it too was showing the login screen.
I logged the original user back in, rather than myself, &amp; we were up &amp; running again.  

I rebooted at that point, normally, &amp; the issue hasn't recurred.

I still have no explanation as to what caused it, but I hope this might help anyone else who sees a similar issue.