How do you open a bonnet on a Fiat Palio Go 2006 when the latch is broken?

The latch is broken, not the cable. You can still get to the cable where it is attached to the latch in the foot-well of the car. Grab the cable with a small vice-grip (or similar) and pull. Note that it is very unlikely the cable will break -the latch typically breaks before the cable since the latch is made from plastic and the cable from steel wire.