Bottom rail of wood panel door falling off
Hard to say whether it's a tenon or a few dowels, but assuming the wood is sound, you can pull the rail into place with a clamp (or a strap if you're low on 8' clamps) and put a couple of 8-10" screws through the stile into the rail. (Plenty of skinny long screws these days.) Predrill with a slightly smaller bit. Do a bit of a counterbore and you'll be able to hide the screw heads with filler.

And if you haven't, paint the top/bottom of the door to seal it.