I haven't used Sonar for a few months.  I just tried to put a vocal on a track, but when I tried to choose an Input driver for the track (my sound card is a Lexicon Omega),  - Input>Selected Track Inputs - there are no choices in the drop down boxes. Also, in Edit>Preferences>Devices>Input Drivers, the drop-down boxes have nothing in them either.  Should not both of these have Omega listed somewhere?  I checked and the cable is plugged in both the Omega and computer, and it always worked before.  In any case, Sonar is not recognizing my card.
 
I now have X3

What OS are you running?
I suggest reinstall of your drivers from here:
http://www.lexiconpro.com/en/products/omega
 

You may have to switch to or from ASIO, or your recording rate (44.1, 48, etc.) may be mismatched.  Also check to see if Windows has recognized your audio device.  It may need a PC restart if you just plugged it in.
 
Brian

I have installed the drivers, rebooted, and changed (as suggested) to ASIO.  Nothing has made any difference.  I can see the driver listed in Control Panel>Programs on my computer, but still nothing shows up anywhere in X2 or X3.
 
 
My operating system is Windows 7, and I now have X3a (I don't see where to change/up-date such info - my profile [?] at the bottom of my post). 

What drivers did you install? Make sure their the right ones for your OS and Architecture of your OS
 
Cj

The driver was the only one offered by Lexicon for the Omega.  Was not a problem before.

Try unplugging the Omega from your computer and plug it back in.

Just tried unplugging for 1 minute, and plugging back in.  No difference.

Go to the control panel in Windows and Programs and Features to uninstall your Lexicon driver. Unplug the Lexicon and then reinstall the drivers. Then plug it back in. 
 
In Sonar select WMD as the driver model and see what is listed.  Be sure that you are running Sonar X3 as administrator. Also upgrade to X3d. 

something are "funny" with USB, my mini korg 61 was.. so maybe the same here.. unplug it.. uninstall the drivers..
 
run ccleaner http://www.filehippo.com/download_ccleaner/  and registry tab, repeat until zero errors..
 
reboot without card plugged in.. install drivers off website.. then plug in... also make your default windows soundcard ya onboard e.g. realtek.. and not the lexicon
 
I've had issues with soundcards on that front over the years.. it a lot of the time boils down to plugging it into a different USB port and the drivers assuming it was in the original one
 
also you may see ya realtek or other card listed in sonar.. that needs unticking input / output wise to be able to select asio