Aaargh. I loaded in Room EQ Wizard and tried to get the loopback test working. Something went off the rails, and now nothing will play in X3 Studio. None of my old projects, even a brand new one won't play without the message "Unable to open audio playback device. Device may not support the current project's audio format, or may be in use."
I can play anything on Windows Media player or internet radio successfully thru the 2i2, but nothing via Sonar. 
I see that all my old projects are 44.1 x 32 (I guess the 32 was not necessary, but was suggested long ago by a friend, and just left it set there at 32, without any issues all this time). Going into Edit - Preferences - Driver Settings, my sample rate is 44.1. But when I go into the ASIO panel button, it comes back with ASIO default sample rate = 48000.  I change it to 44.1, but it doesn't "hold" the change...  it keeps going back to 48000.  Could this be the issue?  Why can't I get it to stay at 44.1?  Did running REWizard have anything to do with this...  everything was fine till I tried REW. 
I tried to uncheck "Share Drivers with other programs" to no avail. Even if I change the sample rate in Driver Settings to 48K to match the rate in the ASIO panel, it still won't play any projects. 
Help !!!!

Have you tried changing the Windows Default Audio Device to anything other than the audio interface?  I have had issues in the past when I tried assigning the ASIO drivers of whatever my audio interface was to Sonar, and at the same time to Windows.
 
Anytime a Windows sound would be made, I would lose Sonar, if memory serves.
 
Anyways - it is simple enough to pick something else for Windows  - like an onboard sound chip, or an HDMI port if you use an HDTV as a display monitor. Then, only Sonar would be trying to access the ASIO drivers of the audio interface.  For MY situation, this instantly fixed everything, and I have kept things separate ever since (plus, I turned off Windows sounds).
 
Bob Bone
 

Turned out I had to reload the 2i2 driver, plus it was a newer version, so that sorted things out.   I previously downloaded the new driver and tried to update my driver to see if it would fix this problem, but the system said the driver was the latest version...  so I didn't force an update, which was a mistake.