I want to move a light fixture 6 feet away for better lighting. I added 6feet of cable and reconnect the fixture .To my surprise ,it did not work . What's wrong?

Okay, can you spend some time telling us what you did or are we to just guess?

Was it wired through a wall switch? It may have been a switch loop and you connected it like a through wire. Reconnect the old and see how the wire colors are connected.

How did you add the cable?
Considering how little information you have given us your screen name is the best answer at this point.

Are you saying you added a 6" extension cord?