It is possible to open the bonnet from below with a bit of care. Obviously a ramp and a good tool kit are useful. Mine was opened by a good independant garage in less than 10 minutes.
When you get it open you might find that it is in fact not broken but the cable has popped out of the release mechanism. This is actually easy to do if your working on the engine and accidentally pull out the release cable without noticing. Then you slam the bonnet shut and find out next time you try adn open it that the release handle is all floppy like a broken cable.
I have actually modified mine with a plate covering the release cable slot so that it can't be knocked out.
Fitting and removing the plastic engine cover easily dislodges the cable as I found to my cost.