We had new drain pipe installed under our double kitchen sink (and the garbage disposal was taken out) several months ago. I have found that a threaded ring screws onto the descending pipe below it, but it doesn’t seem to be connected to the drain assembly above it, just beneath the underside of the (right-hand) sink. Of course, water drains right out into the cabinet area under the sink. What, if anything, is missing here?
I’m confused. If the ring is still on one pipe and the threads are on the other, just reconnect them. If there’s something I’m missing, a picture would help, but I have a sneaking suspicion you have a minor plumbing project on your hands for the weekend.
Wait, I get it. Assuming the tail piece (the part that comes down from the drain) is flared out at the bottom, you need to disconnect it from the drain (either by unscrewing it or cutting it), unscrew the ring, slide the ring over the pipe, put the tail piece back on (either by screwing back to the drain or gluing it on with a coupler*) and then screw the ring back to the next section, which I assume is a trap.
Not a big deal, but whoever did that messed up.
IOW, this should come off the sink, you need to put it back where the ring back where it belongs.
You also need to make sure that washer/flange is there (if the tailpiece doesn’t flare out at the bottom).
A few things could have happened,
1)The plumber may have just plain screwed up.
2)The ‘bottom part’ may have fallen and pulled away from the top part.
3)The tailpiece was cut to short (this goes back to #1).
4)The plumber used the wrong tailpiece and just shoved it all together. In that case, you need to rebuild it properly.
Maybe some other things. Come to think of it, if there’s enough ‘slack’ in the bottom part, you might be able to loosen the nut, take it off, slide it back on to the top part, slide the washer back on and reassemble it. But that will only work if the tailpiece is long enough.
If it was me (and I’m properly understanding what’s going on here), I’d start by what I said just above, but assume I’d have to rebuild the section from the drain to the trap. Not a big deal, but a project for someone not familiar with basic plumbing.
Sounds to me that the compression ring - a washer if you will - is missing from the tail piece of the sink that connects to your plumbing to the p-trap (depending on where your disposal is and stuff).
The compression ring is just that. A small ring of plastic that would fit over the tail pipe from the sink. It has a bevel to it. It may still be in the fitting that fell off.
Well, I saw something that apparently fell off when I looked inside the threaded ring at the top of the (lower) drainpipe. A rubber piece much like a rubber band.
I just took some pictures with my Smartphone but I don’t know how to make them available here.
Can you get them from your phone to pc?, or even direct to imgur from the phone: (http://imgur.com/)
and then share the image link it generates here.
yeah thats the problem, the band has come out of place. The lower ring squashed down onto that rubber to hold the lower pipe by friction (the greater the squash , the greater the grip… )
So you undo the lower ring totally from the upper pipe, and put it on the lower pipe
Put the seal (rubber band thingy) onto the end of the lower pipe, nice and flat and level,and ensure it doesn’t become twisted, lumpy or skewiff
Push the lower pipe up to the top , and screw the lower ring up… tight now ?
The handyman who replaced the uprights supporting our carport roof corrected the problem. He took a threaded fitting out, put it back in, and threaded it closed. Nothing to it–for him. 
Next time it happens, you will be able to do it yourself We used to learn this stuff as children from watching our fathers.