I've had this happen a few times.  Randomly sonar tells me I no longer have a sound card.  I have to restart sonar to get it to recognize again.
 
However after this happens I have to remap all of my inputs again and it's quite a pain because I am recording a full drumset, room mics, guitars,etc.
 
Two questions:
1) why is this happening and how to fix it
2) Can I save my input settings and restore them somehow when this happens again?

Have you tried disabling power management for your USB hub controller in device manager and seeing if this makes a difference? 
 
Look in Device Manager, under Universal Serial Bus Controllers.  Look for anything with the word "hub" therein.... right click and select Properties and then head over to the Power Management tab and disable "Allow the computer to turn off this device to save power"

Thanks, I will try that out

You might want to try searching for issues with the Tascam US-16x08. There were a number of posts regarding similar problems.