My cakes rose just beautiful but then it just sunk, what do I do with all this cake
It appears that they were not completely cooked and I don't think they can be saved. If they seem to be well enough baked, you can still eat them or use them to make cake pops, trifles, etc.
This has never happened to me, the top cooks but not bottom
Perhaps the bottom element in your oven is burned out????