sometimes its the spring on the bonnet, its either not extended enough or extended too much, u need a flathead screwdriver to fix that. anotherpossibility is that the latch cable that runs from the front of the car around the left side of the engine bay into the car has come loose (usually on the front left of the car) just ziptie it so that it runs smoothe wid the engine bay like the rest of the cable and u shud b sweet. easiest way of knowing is look into the hole dead centre front of the car where the bonnet pin goes into, if u can see a piece of wire run thru it, itl click in, if not you need to adjust the cable mate.
ave fun