I am a newbie... I have a Roland a-500s, and trying to use Sonar LE. I DID have it up and running just fine. But then I used other programs for my job. (several!) Now, when I go back to Sonar LE, I cannot get sound coming out of my computer using the A-500s and Sonar LE. I DO get a midi file playing just fine out of Sonar LE. What am I missing?

A-500 is a MIDI-controller keyboard, so it can not produce sound. You need to use a soft synth in order to hear something.
MIDI is like electronic notation. You need to have something to play the notation.

The MIDI-file playback you hear comes most likely from the computers internal Wavetable synth.
The basic idea is: You have a MIDI track with input from your keyboard and output to a soft synth, and an audio track with input from the soft synth and output to your sound card.

I don't know what soft synths you got. Sometimes A-500) is sold with SONAR Production Pack, which includes soft synths, the basic LE is very sparse.