Ive changed the light fitting in my house and now the lifht won't turn off what could be wrong

You wired something wrong. You should wire it back exactly like the old one was.

Sara,

I'm going to guess that this is a single light circuit, controlled by a wall switch. And that the source of power is at the light outlet box in the ceiling. If all of the preceding is true, then most likely you have miss wired the switch loop connections.

Turn the circuit breaker for this circuit off before doing anything else!

I need you to isolate the power supply cable and the switch loop cable.

Once you have done that, place a piece of black tape around the white wire that goes to the wall switch. Next, connect this white conductor to the black supply conductor.

Take the black conductor from the switch and connect it to the black from your new light. Take the white from the new light and connect it to the white in the supply cable.

Make sure you connect the ground on the light to the ground from the supply cable.

Turn the breaker back on and let me know if it works or not.