[DKT1]

The bonnet catch is made up of cone/bolt & spring, as in the picture.
The reason the bonnet wont release is because the cone part has gone past the tube it sits in and is jambed on the edge.
You will have to push it back so it goes inside the tube, then the bonnet release cable will let the bonnet open.
This can occur if the bonnet is slammed hard down or even stood on, to stop it happening again the catch needs to be adjusted so it is shorter, undo the lock nut and do up the cone/bolt until it is too short and it wont catch, lengthen it a half turn at a time until it catches then half a turn more to give it some play is usually about right, then tighten the lock nut.