Here are a few things to try.
Get someone to push down on the bonnet while you pull the release lever.
If that doesn't work, pull the lever to expose the wire inside the cable then, using a good pair of pliers or molegrips, pull the wire.
If still no success you could unbolt the lever and pull it.
You might be able to reach up from under the front of the car and work the catch with your hand or a pole, depending on what spoilers or under trays are fitted.
As a last resort, tear off or smash the plastic grille on the front of the car just beneath the bonnet catch and open it manually. You can easily get a cheap replacement grille from a scrapyard, they're only held on with a couple of bolts.
When you get the bonnet up adjust the cable by slackening the clamp which holds it to the slam panel, pulling the cable tight-ish ( leave a little slack to make sure the catch can close securely) then re-tighten the clamp.
bobo
April 2009