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KDE hang? (not computer, not X11?)

Hi all,
   I experienced a rather weird hang yesterday which logically seemed like a KDE problem but I'm unsure. No long-term lasting problems resulted but I wondered how to debug it or just best deal with it if it happens again. Thanks in advance for any ideas.

   The machine is Gentoo, mostly stable with kde-meta-16.08.3 installed and generally updated every couple of days or so.

   The symptoms of the event were:

1) The machine was up all day. I had been running 2 Virtualbox Windows VMs and had Matlab running in Linux, along with Chrome and other smaller apps. The machine was fine all day.

2) At 2PM local time the stock market closed so I shut down everything except 1 Windows VM which downloads data in the afternoon.

3) At some point KDE became non-responsive to either the mouse or keyboard. The mouse worked, the cursor moved, but nothing responded. The upper-left trigger to see all windows didn't work, the KDE menu was non-responsive and the Windows VM was non-responsive. However the programs running in the VM were still working and completed normally. I just had no apparent way to use KDE.

4) I was able to shell to the console using Ctrl-Alt-F1 where I could log in as root and also in my user account. Ctrl-Alt-F7 brought me back to X11 which was still non-responsive.

5) In the console I was able to communicate with the Windows VM and shut it down using "VBoxManage controlvm"

6) Nothing I could figure out ever got KDE to start responding again. I did a reboot as root and was able to use the machine again. There was nothing interesting that I could find in any of the log files at the time.

   My question is was there something I might have tried to regain control of KDE instead of having to do the reboot? One thing I thought of after the event was maybe hot plug the mouse and/or keyboard to see if that work anything up.

   Any ideas appreciated.