I've occasionally had the experience of not getting any audio when I open a project and sometimes a message will pop up regarding my interface not being recognized, by a specific track or in general. Generally, either closing/reopening SONAR or power cycling the interface will resolve it. This time, I'm out of cards to play, as none of those attempts are working. I can hear my guitar barely coming out of my monitors and there's no input registering in TH3. I opened a project with only 3 tracks on it (each one running an instance of TH3). The inputs on each track are now set to "None" and when I select "Assign Series of Inputs", the dropdown menu labeled "First Input to Assign" has no options in it. I'm probably overlooking something uber-obvious. Can someone clue a brutha?
 

Hi. Do you have sound if you plug your guitar into your UR44, and listen to your monitors / headphones (without SONAR)? If you do, maybe try to change your USB cable, or port? If you are using an USB hub, try to switch to a MB port. Do you have the latest Firmware and drivers for your UR44? Is Windows up to date? That is all I had for now.
All the best.

Make sure power management is off for your usb devices.

I had a similar incident recently and it was baffling me as I was for sure something was wrong in sonar itself.
turned out one of my front hub USB ports wore out. the pins inside that make the connection to the USB input got lose over time (im sure plugging/ unplugging a USB port in to it over 8000 times a year didn't help)
I switched ports, started using the left one now and I haven't had the connection issue again.
 
might not be your problem but there is something to be said for looking in to the obvious.
something I sometimes forget to do.

chuckebaby
I had a similar incident recently and it was baffling me as I was for sure something was wrong in sonar itself.
turned out one of my front hub USB ports wore out. the pins inside that make the connection to the USB input got lose over time (im sure plugging/ unplugging a USB port in to it over 8000 times a year didn't help)
I switched ports, started using the left one now and I haven't had the connection issue again.
 
might not be your problem but there is something to be said for looking in to the obvious.
something I sometimes forget to do.

+1 Chuck.  
Connections due wear out.  As you say, obvious things are often overlooked.

First off - thank you for all the responses.
Zargg71 - Yes, I get faint output in the monitors when I play my guitar. I downloaded the latest drivers and Steinberg confirmed the latest firmware. I switched to another USB port and got the message that the output was not routed to a device. I selected the UR44 in the checkboxes, but still no guitar signal going through TH3 and no playback sound, but I can see the meters dancing. I don't do Windows updates b/c I only take this PC online for the CCC updates.
Gustabo - I looked around in the Device Manager, but don't see anything related specifically to USB devices or the UR44 by name. Are you able to offer any greater specificity? Thank you.
Chucke and FCC - Outside of moving the USB cable to another port yesterday, I've never touched it since the original install over a year ago. I dd notice that the small USB connector, that plugs into the UR44, is a little loose and wiggly in the receptacle jack. Whenever I unplug it at that end, SONAR recognizes it and throws a message window up, so I'm not sure if it's that or some setting that's gone south on me.
 
 

I also noticed that when I play a project, the track meters are jumping up and down as expected, but there's nothing on the Master & the track outputs are routed to the Master.
 

You jumping all over the place. The way to trouble shoot is to methodically start and the beginning work you way down the signal path and eliminate where the issue is. Jumping around and talking about Windows Drivers is nuts. Take a deep breath and start at the beginning. Sometimes it's as simple as a patch cord. 
You said you only hearing a low level at the interface, this points to the issue being there, not anything to do with your computer. 
As said above, plug something in and listen at the headphones or back output with the monitor settings adjusted properly. 
You should hear something LOUD as this is done with the analog part of your interface. 
If it is not LOUD then check the guitar works when plugged into something else. 
If the guitar and cable are OK then it has to be something wrong with the HARDWARE of the interface. UNLESS-
The interface uses a GUI mixer then look there.  
If it is LOUD then you carry on checking the USB and the drivers and so on down the line. 

twanghang
I also noticed that when I play a project, the track meters are jumping up and down as expected, but there's nothing on the Master & the track outputs are routed to the Master.
 

Try creating a new bus and route all your tracks to it. If it works, delete the old Master and rename the new bus Master.

USB Power Management
Control Panel > Power Options > Change Plan Settings > Change advanced power settings > USB Settings > USB selective suspend setting > Set it to disabled

Also
Control Panel > Device Manager > Universal Serial Bus controllers > One at a time, right click on each device there, select Properties, Click on Power Management tab (if one exists) and make sure "Allow the computer to turn off this device to save power" is unchecked