Couple of quick thoughts.
I have 2 flashes, 420EX and 220EX. Noticed the other day with the 420 the flash is not showing up in my pics - I see the flash burst in the view finder but my target is not "flashed". Take off the 420 and put on the 220, presto. Changed the batteries (thinking maybe it was having an effect but no difference.) Any thoughts?
Make sure you didn't changed the flash setting to Not Fire by mistake or something.