Canon S900 - No black ink (none!)

Here's a weird one. Read others with S900 ink problems. Tried a few suggested soloutions such as replacing ink tanks, cleaning print head etc. Even tried reseting waste ink monitor (not getting this error msg but thought I'd try anything). Doesn't matter what I print, I get no black ink. I mean NONE. Nozzel check pattern = doesn't even put a speck of black in on the page. Thing is, it just stopped all of a sudden. Even moved the black tank to another spot (know I shouldn't do this but figured what the heck). Prints perfect pattern in black there so it's not the tank. Always used Canon inks. Cleaning print head I can get water thru the black nozzel about the same as all the others (not very easy).

Suggestions?