I can set up a DM Pro track, setup a midi to play it. It works fine- I can hear it when I use my keyboard controller. I walk away to eat lunch, I come back and it makes no sound when I play the keyboard controller BUT if I go ahead and record something (can't hear what I'm playing) it will play that back just fine.  When I hit the keys on the controller the meter responds. But no sound except for playback.
Anyone have a clue what's going on? 
 
Win 7
XP2
M-Audio Midiman 2x2
Presonus Firebox
DM Pro 1.5 

I just did a quick check of other synths and the same thing applies. And using a virtual keyboard like Dropzone has also yields no sound when I play it.

Is it possible that Windows is disconnecting USB devices?
 
The default Windows behavior for USB devices is to selectively suspend them after some period of inactivity.
 
You can check/change this:
 
1.  Control Panel\Hardware and Sound\Power Options
2.  Click to select the High Performance Plan.
3.  Click on Change plan settings, next to High Performance
4.  Edit Plan Settings (make sure you choose the High Performance power plan).
5.  I recommend you set it to never let the computer go to sleep.  Then, click on Change advanced plan settings.
6.  In the Power Options dialog box, expand USB Settings, then expand USB Selective Suspend and set it to Disabled.
7.  Click Apply, then back you way out - if there is a button to save your plan changes do so.
 
While you are in the Power Options settings. I also recommend you look at and change, if need be - and if it exists for your installation of Windows, another setting.  This one is under Processor Power Management, and under that within Minimum Processor State - set this to 100%, to have your CPU run at 100% all the time.  This is good for Sonar performance.
 
So, please review and consider applying the above, and post back with results after making the changes and then loading a project and waking away for a bit - like you described in your initial post.
 
Thanks, 
 
Bob Bone
 

It's firewire. Does it do that to it as well?

Hmmm --- check the 'input echo' icon on the track and at the top of the screen - you might have disabled it.

Firewire does not do that.  Please ignore my earlier post - I should have asked if you had USB - I did think about it, but was too lazy to type it in.  Sorry :)
 
Bob Bone
 

Wait - I was referring to your keyboard controllers.  How are they connected to your computer?  If USB, then please do check the earlier info I posted.
 
Bob Bone
 
 

I made all those changes. And I'm keeping my fingers crossed. I just realized that the midi is USB. I was thinking of the Firebox. Thank you for your help. I have a feeling that this may be the answer.
 

I believe the Midiman is USB?
At any rate, I have seen "Input Echo" get "lost" after a period of inactivity. Usually toggling it through an on/off cycle will bring it back.

Actually, it is probably not the answer. If the keyboard controller were not connecting to the computer because the USB port was powered off, there would be no meter action and nothing would be recorded. More likely this has to do with Sonar losing control of the driver for the audio interface.