A few moments ago I had a problem.
I opened a project that I had been working on and when I hit play there was no sound. I haven't worked on it for a while and made no changes since the last time I worked on it.
 
I made sure nothing was muted and checked my driver settings. Nothing had changed...except for no sound. I loaded a completed project and again...no sound. I figured third time the charm, so I loaded another project.
That one seemed to be working, so by now I thinking WTF.
 
I loaded up my first project and once again...no sound.
On a hunch...I decided to restart my computer and now everything is playing loud and proud.
 
However...I'm still baffled as to what caused the problem in the first place, and what can I do to prevent it in the future.

Check Windows has not made your HDMI audio the default driver. Mine sometimes does that.
 

guitarwiz1
On a hunch...I decided to restart my computer and now everything is playing loud and proud.
 
However...I'm still baffled as to what caused the problem in the first place, and what can I do to prevent it in the future.

Did you install any software prior to experiencing the problem?  Has it been more than a couple of days since your last system reboot?  Do you have any software where automatic updates is enabled?  (ie. Antivirus, iTunes, Adobe Flash)
 
It could have been one of these.  Or none. 
 
Bottom line, these "glitches" are just a fact of life with Windows -- though more likely when you have a lot of background tasks going and/or are connected to the web.
 
If you're experiencing these problems often, there are utilities that can monitor everything on your system and help you troubleshoot any issues.  
 
But if it's not a common occurrence, I really wouldn't worry about it.  Just be glad a reboot gets you back up and running!

Is your soundcard on USB? If so Google USB power options to stop USB ports going into standby mode.

This is the first time that it's happened.
It just seems strange how one of my projects was working correctly, and the others had no sound.
 
It does seem like every time windows has a major update things get a little wonky.
I'll keep an eye on it and see if it happens again.
 
Thanks

guitarwiz1
This is the first time that it's happened.
It just seems strange how one of my projects was working correctly, and the others had no sound.
 
It does seem like every time windows has a major update things get a little wonky.
I'll keep an eye on it and see if it happens again.
 
Thanks

I've had similar issues and put it down to win updates. I've had to re-install my interface drivers a couple of times this year to get things working again. Never do seem to actually get to the bottom of why, particularly why one project, and not another, but, I can live with it....
 
edit - actually I did note a problem between projects that was causing exactly this. I have been working on some material aimed at producing a video, and using 48k rather than the usual 44.1k. When changing between projects, regardless what my interface is currently set to, it causes audio engine dropouts, that I can't turn back on, without exiting and re-booting. Still trying to sort a work pattern that allows me to change between projects without the drama. This only started a few months back and I've been too busy to set aside the time to figure it out. Cheer. Dave.

This has to be something with win 10 and changing sample rate or like you said when going from one project of 48khz to another of 44.1 khz  its freezing the usb disconnecting audio driver.
Peace

When it happens first I would observe the console to see if the interface meters are showing action. 
This puts you in the middle between Sonar and the  interface.  
If they are not showing a signal then work backwards into Sonar. = Master buss output-Audio engine, preferences etc. 
If they are showing a signal work forwards. = Windows audio settings, interface control panel