Check the cmyk values for your black in the color palete - you may have a black made up from cmy(k) with not much or no k
Also, you might check the document colour mode is cmyk
You really haven't offered much information.
What is the exact color break down of your "black" -- K only, RGB? CMYK?
What is the document color mode?
What are you printing to?
I had the same issue recently and for me it was my CMYK values. I find that C 0 M 0 Y 40 K 100 is a great combination to ensure your document prints black.
Look in the color palette when you click on your black to see what the values are and then adjust accordingly.
There was a lengthy discussion a little while back about what "rich black" means. If the forum search wasn't so abysmal, I would track it down for you, but the bottom line is there is no official definition, it's based on the application. If your art is yellow, then yeah, throw a bunch of yellow in there. If your design is pink, a yellow-black like that might not work so well.
The only constraint you have is the total ink amount, which should never exceed the amount defined by your printer. (and if it does, they will back it down, sometimes without telling you.)
Actually everyone is guessing at what you mean can you tell us about the document it might be as simple as selecting a spot black instead of the process black to avoid the problem or a solid black separating.
Also check the your preferences to see what the appearance of black is set at.