I removed an old light fixture months ago, and finally found a new one to replace it.
My junction box has white,black,red and copper ground wire. But the new fixture is a chandelier. It has a white,black and ground wire. I attached the black, on new light, to the red in junction box,thinking it was for the switch. But it doesn't turn on. What do I do?

How was the old light connected?
With only black, white and red you don't really have too many options.

I agree with Stan, you don't have a lot of choices here. My thinking is that maybe the ceiling box was wired for a possible ceiling fan/light combo.
In this case the black wire would be the hot for the light and the red wire the hot for the fan. (not the complete story, but OK for our purposes)

So, I would hook Light White to Ceiling White, hook Light Black to Ceiling Black, and hook Light Ground to Ceiling Ground. Then, cap or tape off the red wire completely.

As with anything electrical, be extra cautious.

Hope this helps,
WallyH

Is there possibly a second switch in or near this room that also controls the light? You might be dealing with a three way switch. Check your existing switch, do you see a clearly embossed ON and OFF on the toggle?

I agree. I think there was probably a ceiling fan there before. Match all the like color wires ( copper/copper , black/black, white /white and cap off the red. It should work.