I have a field recording that is 2G in size on a single stereo track. 
 
When I open the project initially I get playback as expected without issue.
 
I had a break and let it sit for about 2 hours. When I came back, SONAR was still open and the project was still loaded. Playback would not come audibly. I see the meters move, but no audio. Bouncing SONAR results in playback again. 
 
My question is what is at fault here? Sonar or the audio driver?  This has happened 3 times this evening with variations of the open state time of the project.  Sometimes stuff comes up and I let SONAR sit. It would be nice if it played when I returned.
 
No power saving issues. I have all of that turned off.  So nothing goes into a power saving mode.
 
Thoughts?

Is the file on an external drive that could have gone to sleep?
I have had problems with Sonar locking up after sitting for a while...Don't know why

Guitarmech111
So nothing goes into a power saving mode.

 
Even the USB controller hubs that you access in the Device Manager?
 

Thanks for replying guys.
No external drive. All internal. Computer is not allowed to turn off the USB ports.

FAT32 or NTFS audio data drive? FAT32 has a file size limitation that might be 2G (can't remember) that might cause some funkiness.

What happens if you set it playing and leave it playing without touching the computer?  Does it still hang up?

A memory leak?  Next time it loses playback, check memory usage in task manager.

It is NTFS.
 
I thought about a memory leak too. 
 
There is only 78 mins of data in the file. I let it sit more than that on all failing occasions.
 
Thanks all!

It's not clear that the big file has anything to do with the problem. Is it only this one project?
 
If the meters are active but there's no audio, I would suspect there's a driver state problem; SONAR thinks the port is open, but it's not. Cycling the audio engine by the button in the transport module should be sufficient to reset the audio driver. And you might try enabling Always Stream Audio Through FX in preferences if it isn't already and insert at least one plugin in the project to keep the audio engine and driver active.
 
 
 
 

brundlefly
And you might try enabling Always Stream Audio Through FX in preferences if it isn't already and insert at least one plugin in the project to keep the audio engine and driver active.

 
Creative thinking! I love it.