Your dwarves will automatically take a fresh stone out to the stonefall trap and reload it, provided they can reach the trap and the trap hasn't been forbidden.
The invader bodies and loot may be forbidden by default. Un-forbid them to have them moved to a stockpile.
Cage traps will be reloaded, provided you have empty caged and idle dwarves who can reach the traps.
Annendum:
- if you accidentally/intentionally forbade the trap (either by using the (d)esignation options, or by forbidding what they are made from using (t)), they will not reload
- When an enemy dies, all of their loot is forbidden by default. There are a few ways to fix that - use (k) to look at the spot, and press (f) as you select an item to forbid/unforbid it; Or, use mass designation - press (d), and follow it from there. I can't remember the exact one...; or, use stocks to find items you want and unforbid them.
- With many traps, only dwarves with the Mechanic labour enabled will reload them
- Dwarves can and WILL try to reload traps in the middle of an invasion. Be careful.
- Consider investing in Weapon Traps as stone-fall traps quickly become ineffective. You would want metal trap components or weapons, or any material crossbows. Crossbows require bolts. You can place up to 10 weapons in each trap. Those traps can get jammed.