Yes, arnisador, part of the rules state that if your mawashi comes off, you lose the match. The reason you don't see this anymore is because if a rikishi's mawashi is getting dangerously loose, the referee can stop the match to fix it. I've see a few stoppages before, usually because it's going really long but in one match this last basho someone got a bloody nose. The ref taps both rikishi on the back and they stop. He then checks out their positions (feet, hands, arms, bodies, heads, everything) so he can get them back in the same spot again, and then lets them separate. Once apart they can get water and towel off if it's a break in a really long match or fix whatever the problem may be, say that super-loose mawashi.
There are a number of illegal techniques that also cause you to lose the match. I forget all of them offhand but they include clapping both of the opponent's ears at the same time, punching with a closed fist (almost certain), I think slapping the face (neck is OK though), and grabbing the vertical part of the mawashi (for fairly obvious reasons). If you want to know all of the illegal techniques I can look them up for you later.