I made a chocolate pudding pie topped with meringue. and the pudding turned out incredibly runny, any idea why?

i dont know what i did wrong? i used the cook and serve jello pudding and did it acording to directions. then i put it in the pie crust and topped it with the meringue i made. and put it in the oven. im thinking that the heat from the oven messed it up. but i know its possible to do it right because i have eaten it at a party before