The cable may be broken or worse. Have you ever lubricated it? your very lucky most broken hood latches are the hood is stuck closed and that is not fun. Have to keep the latches and hinges lubricated. Take some pictures of it And post them.
Lubricate the latch (WD-40, oil, etc) and try it. I have seen the latch and cable damaged by rust and ended up having to replace both of them. Posted via the PriusChat mobile app.
Are there rubber "bumpers" the hood lands on? The kind that screw in/out to adjust their height? Long shot, but maybe one or more of those is set too high?
Was it able to close previously, or is this a new (to you) car which has this problem? Was the car just in an accident? Does the latch "not close" because it does not reach its target (or is otherwise displaced) or because it does not move? Is the latch already closed? That may seem like a strange question, but if it has been triggered to close with a screw driver or some other object it cannot then engage its normal target until the hood release is pulled. If this is the case when the hood comes down there will be a load bang when the two pieces of metal collide. Depending on its design a broken release cable might on some cars cause the latch to close. I do not know if this is the case on a Prius.