All of a sudden, I can't play mp3s, videos, or any other sound on my PC. I've tried 2 different browsers, but no help. However, I can fire up Sonar Platinum and the audio works fine. I'm sure it's something simple, but I'm at a loss. Any guesses?

I assume you're using ASIO? If so that would explain it. IIRC you have two options...
 
1. Run SONAR using a Windows driver mode, like WDM.
2. Don't know what version of Windows you're using, but go into the Windows audio settings and set the Focusrite as the default for Windows.
 
Circle back and let me know if that helps.
 
Remember that the big advantage of using Windows drivers is that, like the Mac's Core Audio, they can aggregate interfaces and run sound for different programs simultaneously. However ASIO will usually give a performance edge (YMMV) with high-performance audio applications like SONAR, so you have to decide which is more relevant to your needs.

I was using the same ASIO drivers, as furnished by Focusrite, that I've been using since I got the 18i20. They have worked perfectly for all devices up until today. 
I switched over to the WDM driver. The audio within sonar was completely garbled, and still, no other device played. Switched back to ASIO and all is fine within Sonar, and still not working in the Win 10 environment. Very strange...

Did you try what I suggested in (2) above? Much to my chagrin, Windows often resets defaults after an update. 

Sorry. Forgot to mention, yes, it's set to default. It's like something updated, but I see no evidence of that.

I haven't worried about this issue for, well, ever. I use two audio systems. One is for Sonar and other programs that can use a high quality audio interface and the other is for everything else. That one uses the on board audio chip and is quite adequate for those purposes. I've been doing this for so many years I can't say when I started doing this. I use to use a Sound Blaster along side a Tascam digital mixer. That proved to be a very simple solution to the problem of switching on and off my Tascam. Plus the TDIFF card I used had trouble with hanging Windows if it was on when I booted Windows. It became a habit that has proved useful through the years.
 
You need to invest in a separate setup for your speakers but once its done it should not need further upgrades. The other benefit is I don't need to have all my external gear on just to read the Sonar forum.      

John
I haven't worried about this issue for, well, ever. I use two audio systems. One is for Sonar and other programs that can use a high quality audio interface and the other is for everything else. That one uses the on board audio chip and is quite adequate for those purposes. I've been doing this for so many years I can't say when I started doing this. I use to use a Sound Blaster along side a Tascam digital mixer. That proved to be a very simple solution to the problem of switching on and off my Tascam. Plus the TDIFF card I used had trouble with hanging Windows if it was on when I booted Windows. It became a habit that has proved useful through the years.
 
You need to invest in a separate setup for your speakers but once its done it should not need further upgrades. The other benefit is I don't need to have all my external gear on just to read the Sonar forum.      

 
I've had Windows reset my audio to something else. All that's needed is to right click on the speaker icon located in the tooltip area of the windows toolbar, right-click it, and select playback devices. Make sure your audio devices is set as default.
 
I differ from John in that I use my monitors for Windows audio as well as ASIO. My driver supports concurrent ASIO and WDM. 

@John, That would sure solve this PITA issue. It's just frustrating that it has worked flawlessly, up until today. The other issue that would be resolved with two systems is my monitors are powered, and they shut off when the don't thinks there's sufficient signal being supplied. Anyway, I just so happen to have another set of speakers....

@SuperG, Did that, and it's set correctly. Thanks for the right-click hint. I always went in thru the Control Panel. :)

I have similar problems, but only when I run sonar at 96K.  If I run at it 41 or 48K then everything works fine outside of sonar. But if I am running sonar at 96K, then exit sonar normally, I don't hear any other sounds.
 
I am on a windows 10 16G, I7.
 
But concerning 96k, fortunately, if I reboot my system I regain access to all teh rest of my computer's sounds.
 
Until the next time I run sonar at 96K.  Needless to say I usually run it at 41K.
 
Rick
 

That's a great hint, @rickidoo. In part of the troubleshooting process, I set the sample rate to 96k. However, when I checked in now, it was at 41 k. Strange