19: Losing from the bottom
Once your opponent gets past your legs, you're losing. His goal will be to hold you down for 25 seconds or submit you with an armbar or choke, and since he's on top of you, he will likely be successful.
If you have any sense, you will try to do one of two things: scramble to your feet or knees, or get him back between your legs. In order to do either, you'll need to escape his hold down.
There is a third thing I often see judo fighters do, though: turtle. If you are turtled, as far as I'm concerned, you've lost entirely. I think that turtled fighters should automatically lose any competition.
My rationale is simple: judo was meant for fighting. If you turtle, you are abusing the rules and stalling, in the hope that someone else (the ref) will intervene. In real fighting, there is no ref. There can be no stalling, and there are almost no rules. When you turtle, you are allowing your opponent the chance to stand up, step back, and football kick your teeth to Texas.
Turtling is unsportsmanlike. Don't be an unsportsman.
If you are held down, there are a couple of tricks. First, make some space. Whatever hold down you're in, you need space. Using your chest and arm muscles is terribly inefficient; few people are strong enough to bench press an opponent off them. If you can, buck your hips or work your knees in.
Second, turn toward your opponent. Turning away makes it impossible to make space, and lets your opponent close the distance you might have had.