Fallen Meringue

Some meringues are more stable than others. They're all going to start weeping and looking a bit icky eventually - but should hold up for a few hours regardless. Perhaps you over-whipped your egg whites? That can lead to a cottage cheese-like consistency and weeping moisture.

I don't have a copy of the recipe you used, but if you search for an "italian meringue" recipe, that's a very stable form to use and holds up for a good day or more. Hot sugar cooks the egg whites in a stand mixer, no baking required. (All the meringue pies I've ever made haven't been baked - just torched for the cooked marshmallow effect.)

It sounds like the meringue wasn't cooked long enough, leaving it raw in the middle and prone to collapse. You want to make sure your filling is absolutely piping hot when you spoon the meringue on top. Pop it in the oven immediately. You'll want to bake it at a lower temperature (325) for a longer time (20 to 30 minutes or so). You know how when you make free-standing meringue cookies, you have to bake them low and slow? Same principle here--there's a lot of moisture in meringue, and you want that to cook out without the meringue burning. If you pile the meringue very tall on the pie, you'll have more trouble cooking it through.