Where is the bonnet release on an SC430? If it's near the floor you may be able to do this to get in:
I suspect the problem you have is a weak battery, failing that it could be a fault with the security system but weak battery is probably more likely.
If you can get into the car without damaging anything you can test the security system by turning the key to position 2 in the ignition, if the security light goes out then your key is recognised. Then I suspect you'll need to charge the battery somehow, best way would probably be to get a jumpstarter from halfords or amazon and then drive it around for a bit to charge the battery.