My main Midi input is a Yamaha P-105 keyboard. I am using the USB input, though I also have a Midisport box for my Roland rack mount synth. I use the Yamaha for most of my piano tracks, EastWest for orchestral stuff, and the Roland synth for everything else.
 
Sometimes, no idea why, the Yamaha will decide not to play or record. I have discovered that changing the instrument on the hardware Yamaha's buttons (and then back to the instrument I want, a quick tap-tap) "wakes it up" and it will play back. Sometimes I have to do this every time I playback or record. Sometimes it works just fine. Drives me crazy. My previous keyboard, a Kurzweil with Midi in/out instead of USB, never had this problem. Is this because I'm using USB? Should I switch back to the Midi card? Is USB input for Midi controllers a known buggy thing? Honestly, it feels like a problem with the Yamaha, not Sonar, (though my constant struggles with Sonar means that my trust that Cakewalk handles this properly is pretty much zero as well). Thoughts?
 

Do you have power management turned off on your USB ports?
 
With this on Windows will power down or sleep attached devices.

I second the Wookiee. Make sure your USB ports are set to stay on.

What they are referring to is that by default, Windows is set to selectively suspend idle USB devices, after some number of minutes.  If this occurs, Sonar has problems communicating with them.
 
So, there are a couple of ways to change that default Windows behavior, and here is one way:
 
1) Go to Control Panel > Hardware and Sound > Power Options 
2) Choose the High Performance power plan
3) Click on Change Plan Settings
4) Make sure that 'Turn off the display' and 'Put the computer to sleep' are set to Never
5) Click on Change Advanced Power Settings
6) Scroll down and expand the USB Settings options
7) Expand the USB Selective Suspend Setting options
8) Set the USB Selective Suspend Setting option to 'Disabled'
9) Click Apply and back out of everything - IF you also have a button to Save power plan changes, click Save.
 
So, the above tells Windows you want maximum performance all the time, AND that you do not want Windows to suspend anything.
 
Please review the above and give it a shot - it just takes a couple minutes to perform the above step. :)
 
Bob Bone
 

Thanks for the unanimous tip! Turned out that it WAS set to sleep to save power, so I made the switch, also set the Selective Suspend Setting to disabled, rebooted the machine...
 
... no difference. Problem persists.

I had to go all the way into Device Manager and disable power management for everything listed as a 'Hub' - some called Root Hub, some called Generic Hub
 
These are listed under the 'Universal Serial Bus controllers' section
 

Yes I did that. Apparently there was something wrong with the project, because I created a new project instead and copied tracks one by one into it. Took several hours. Thanks Sonar! :-(