Hello,
Sorry to come back with problems but despite the fact I read all the other threads relationned to Sonar Audio Engine drop out, I report here my case which sounds rather strange :
I worked on a project, with just 2 audio tracks, 2 or 3 soft synths, plus testing the Fxpansion Geist demo version (with, every 15 minutes, the Geist demo warning message that sucks !), maybe adding some Midi controller and everything was ok; I made a long rest (maybe 3 hours or more) without shutting down or touch anything, and when come back, I had Audio Engine drop outs every second continuously (with the well known message : the audio engine has stopped suddenly...).There was very little activity in the CPU consomation.  To be sure, I tried some settings recommended by Cakewalk, but it's as if I did nothing ! 
Given everything worked fine before , I closed Sonar (without recording the current settings), reopened it  and I could work normally. Weird, no ? I don't know if it this is directly related to Geist or its demo version, but if you have any idea or experimented similar things ? 
 
Thanks again for your attention
 
Pierre
 
Sonar X3 e
Windows 7, 64 bits, proc I7 970, Ram 12 Gb; Asio roundtrip latency 9.4 ms; buffer size :256 ms...
Very Recent Windows update and reinstallation of the Roland VS700 drivers.

Seems like your computer went into sleep mode.

There might be a power setting shutting down drives or ports. Maybe a background process gets fired during long periods of inactivity.

I may have been experiencing similar faults. I'm curious as to what happens if you mute, one at a time, each track.  I'm thinking that one of my tracks is corrupt (that may be impossible) and causing the Audio Engine to stop.  

There are options in the Advanced Options for Power Management in Control Panel, that control whether or not USB devices are selectively suspended (put to sleep) after some period of inactivity.
 
When Windows DOES put a USB device to sleep, Sonar can definitely get a bit wigged out about it.
 
Most folks go into these settings and tell Windows to NEVER selectively suspend devices or to go to sleep.
 
Until you get a chance to change those settings, if you lose a USB device - if you exit out of Sonar, then unplug and connect the USB device again, it should get detected again by Windows, and then when Sonar fires back up it should also be seen once again in your project.  (please note that it has been AGES since I have had to change those settings - not since I first built this computer 3 years ago - but I believe the above will work for you, though certainly changing the settings would be my preferred advice to you).
 
 
Bob Bone

Thank you very very much to everyone for your help and suggestions !!
I just arrived at home and didn't check my system settings. I'm almost sure that my computer doesn't go in sleep mode because I have not to re-open the session; it's just my screen monitor that turns off in standby mode. BUT I had never problem like this before and always could resume fluently my work on project after long period of inactivity. Maybe, it's because I've just uninstall and reinstall my VS700 drivers and made Windows update (?)...
I have to search towards what Scook and Bob suggested... Thanks Bob for the precisions about the "options in the Advanced Options for Power Management in Control Panel" : I suppose that is where we can manage Power settings ?
 
Cpkoch : Didn't tested the mute operation but sorry if I don't understand well, but can a track corrupt a project ? Could you identify it and could you solve the problem ?