Yep. Tamp it down well with your hands. If it's still really, really loose, you might even try walking on it. The top should be soft enough that you can dig through the top of it with your hands, but not loose enough to scatter if you run your hands through it. An inch or two down, you could dig through it with your fingers if you really try, but hard enough that you don't really want to
Actually, it occurs to me, what happened is the plants were fine, then the lower portion of the stem went hair thin. I don't think falling over is actually the problem, I think the problem is whatever caused half of the stem to become a hair instead of a nice thick tube.
here are a couple of links. see if you think they describe your problem. if so you probably do need to start over. i don't think you csan reverse damping off, but the links have some tips for prevention.