The stop down on my K100D, DS bodies and the K10D look exactly the same to me. Assuming that is the case, I had this happen to me on one of my DS bodies and fixing it was relatively simple:

There is a little spring that hooks over one end (the lower end or the end closest to the bottom of the camera) of the actuator part that slides up and down. The spring keeps the actuator held down until the gear moves the whole assembly up. The spring came unhooked from the stop down actuator.

You can probably see just the end of the spring (it is actually just the straight part, hard to explain) if you look closely next to the actuator near the bottom.

Move the actuator up manually and then using a fine tweezer or maybe the end of a paper clip or something similar, push the spring end up and towards the center of the mount or away from the silver K mount ring, pushing it up around a little protrusion on the bottom of the actuator. It will snap to the left (looking into the camera) around the little protrusion that sticks out on the actuator and then it will hold the actuator down as it should.

Sorry if this is confusing, but if you look at it closely, I think it will be obvious.

Ray