Why an IC is not working?

This question would be easier to answer if the type of IC was specified.

But generally, there are 3 reasons an IC will not work:

1. It is broken. If a large voltage or shock has been applied to it, or the power hooked up to it in reverse, an IC will usually burn out. This may or may not be accompanied with the release of the Magic Blue Smoke. (http://en.wikipedia.org/wiki/Magic_smoke)
2. Your power supply is dead or not outputting enough voltage. If the voltage supplied is too low for the IC to use, it may not function properly or at all. Check the power supply.
3. The circuit the IC is in is broken. Check all the wires/solder points. Test the IC in a known working circuit.