I have a similar issue sometimes. The brush works, but you can't actually see the brush on screen, just a default set of crosshairs. Strange. Rebooting does the trick though.
Yes CAPS LOCK sounds the problem to me. (and you dont have to select a source with the ALT key for the healing brush.. .only the clone tool) I think!
For the healing brush you DO have to ALT click (for sampled mode anyway).
http://www.mediacollege.com/adobe/photoshop/tool/healing-brush.html
You have to select a source point with the healing brush before it will do anything. Alt-click to select a sample.