I've got a similar problem with my Pismo keyboard... the space bar.
It's got a long upside-down-U shaped metal bar, secured by clips at the right and left ends.
Unfortunately, the bar has popped free of the clips at the left side. This means that trying to press the right side of the space bar means it just pivots in place, so there's no pressure to register on the little plunger in the middle.
Pushing really hard on the left side clicks it back into place, good for a couple presses or so before it pops back out.