mine snapped on my old mk2, i needed a new bonnet anyway so i cut into it. I tried to open it using a sort of over sized allon key (don't know name of tool) and with a bit more patience i could probably have got it open.
If is the mk1 then just lie down with head and shoulders under the front end and can put your arm up through and open it. Just make sure the exhaust is cold I forgot after snapped a second one and destroyed a 200 quid jacket
If your release handle in slack maybe your cable snapped off inside. same thing happened to me on my mk2 punto. If u get it on a ramp u can get a small screwdriver in to the bonnet release catch from underneath. Its a bit tricky but if you can find the hole in the side of the catch you can push the springs apart to release the bonnet then all that is left is the safety catch which you normally open from the front. I got a new bonnet release cable from the fiat dealer only cost approx 10 pounds. the cable is longer than needed but i think its for the left hand drive version