freewheel won't 'engage'

Hi all,
My freewheel (9sp Shimano XT) will not engage, unless I push the cogs
towards the centre of the wheel.

i.e.: When I spin the wheel, it doesn't 'click' until I push the cogs. When
i spin the pedals, they cogs spin freely, and the wheel remains motionless.
If I spin the wheel in reverse, it doesn't force the pedals to backpedal.

Any thoughts on what this is? What tools I'll need to pick up to fix it?

Many thanks in advance.