If the problem valve pulls more easily than the other 3, the cable might have come loose from the valve assembly.
It's also possible that something is blocking the drain, but if there's not even a drop of water coming out, that seems less likely.
You may have to drop the coroplast underbelly on one side to locate the gate valve. If you can reach it, or can cut an opening under it, you could reach the plunger and see if it's moving when the handle is pulled. If the cable came out of the plunger, it's not hard to put it back together. Also, if you can reach the plunger, you can probably pull the plunger by hand or with pliers to get it to drain.
This is what they look like.
If you're going to take out some of the screws holding the coroplast to the frame, an impact wrench will work a lot better than a socket wrench.