I have a Trendnet 4 port KVM switch that I've used for years.  Since upgrading to Windows 10 (from 8.1) if I have Sonar full screen and switch to another computer for a while, when I switch back the Sonar machine doesn't respond.  No mouse input and no keyboard input.  I've tried unplugging the USB for keyboard and mouse which doesn't help.  So far my only choice has been to force the computer to power down.  It never locks me out if Sonar isn't running.  I can't tell is it is an input issue or a system lockup.  There are no relevant entries in the event log. Anyone have suggestions for the cause or a fix?

Update it may not be the KVM switch.  Sonar just locked up while I was setting up a new audio track.  I was working on an old file that was created some time ago in an older version of Sonar.  Nothing special it just stopped, no error or dropout message.

Sounds like you have the same KVM switch I have...I also have been using it for years with no issues. 
Have you looked into the Windows event log?  Sometimes that will point to the issue.
 
 
 

I've determined that it is not the KVM switch.  It is either Sonar or a plug-in.  The machine never locks up when Sonar isn't running.  When it does lockup it just freezes and has to be powered off with the power switch.  There are no messages in the event log except startup messages.
 
I've updated the BIOS and updated the Garritan Aria player (was using it at last lockup).  None of that helped, it still randomly locks up.  It's an i7 quad core with 16GB of RAM and an SSD boot drive.  With a recent update to Windows 10 from 8.1.  It's usually disconnected from a network.

Assuming that you have a USB interface, check that the USB port that it connects to is not set to power saving mode.

You can either go into advanced options in Power Management through Control Panel, to set the USB Selective Suspend off, or you can do it from within Device Manager, by double-clicking on each USB hub and then clicking on the Power Management tab and removing the check from the box that allows Windows to turn off that device to save power.  Sonar and devices that get turned off/on do not get along well at all.
 
You also want to be careful to avoid Green disk drives, as strange behaviors have been reported due to something with the variable rotation speed to conserve power.
 
Bob Bone

I get this same issue with mine but narrowed it down to how many days my pc is up. Anything over 4-6 days, and it will lock the one pc to where I have to force a power down. I also get an issue where the KVM won't stay switched on the Sonar machine. But even there, as long as I reboot after 3-4 days, I never have an issue. All my power options are set manually to high performance so nothing is suspending. I tried another KVM switch, same thing happens. It has something to do with up time on my end.
 
-Danny

It's not the switch, but rather software that doesn't re-establish its connection to the reconnected USB port. Your audio interface driver would be the prime suspect (assuming you have a USB-connected interface).
 
I have similar problems if I use the switch while my VirtualBox VM is open. When I come back to the machine running the VM I have no response from the mouse or keyboard. I have to power off and reboot to recover, losing all my data within the VM. Needless to say, I avoid doing that! 
 
I have not seen the same problem with SONAR. But that's because my audio interface is Firewire rather than USB, and not switched. It's only the USB-connected devices that are affected.
 
I do have an occasional issue that could conceivably be related. Sometimes after waking my computer from sleep I have no audio at all. Not from SONAR, not from CDs, DVDs or YouTube videos. I mention this because maybe my workaround might apply to your situation. What I do is open the audio interface's UI (in my case Saffire MixControl) and change the buffer size. That wakes up the driver and/or port, probably because it forces a re-initialization.