Several things may cause this problem:

By default SONAR shows only the audio cards on your system that have WDM drivers installed. Follow the steps to enable SONAR to access all of the audio drivers on your system.

* Start SONAR, click Options | Audio | Advanced Tab and select "Always Use MME..." Close SONAR and restart. You should now see all MME audio devices.
* Enabling this option will force the WDM driver to go through the slower MME interface. If you use only WDM driver devices, leave this option unchecked for optimum performance (lower latency).
* SONAR's AUD.INI file may have become corrupted. 

Easy Fix - Close SONAR and delete the AUD.INI file from the SONAR folder. After restarting SONAR you may need to point the app to your WaveData folder. (Options | Audio | Advanced Tab | Data Directory field)
Advanced Fix - Close SONAR. Open SONAR's AUD.INI file in Notepad. Delete everything listed beneath the line PictureDir=C:\Cakewalk Projects\Picture Cache 
Save the file and restart SONAR.