Had a similar problem with my 325i. In my case the springlike coils connected to the hood cable release that normally would grab the plunger on the hood stayed retracted due to corrosion, stiffness, and accumulation of all sorts of stuff over the years. The temporary, workable cure was kinda simple enough, get the screwdriver from out of the trunk and just flick the coils toward the opening of the plunger holes. Bit of an inconvenience having to do this practically every time the hood was opened though.
A more permanent solution came about when I was having some body work done on the car (replacing rusted sections) and the shop guy got in there and cleaned up and lubricated the release cable and mechanism. You might want to have that done at an auto body shop, I really don't know how to gain access.