Bathroom sink clogged, refilling and overflowing - please help!

Yikes, I really hope someone can help us!

An upstairs bathroom has had a clogged sink for a while - we're super busy with a new baby, and there are other bathrooms in the house so we've unfortunately kind of ignored it for a couple of weeks. Tonight, however, it reared its ugly head and actually overflowed. There was no water in it earlier today and nobody has turned on the tap. (this would seem to explain the old water stain on the ceiling of the room below and the old owners will have some serious karma to deal with, but that's another story entirely...)

We scooped most of the water out and used a plunger on it and some really nasty looking black gunk popped into the sink through the safety drain - looks like slimy stuff that's been building up. There were brief moments that the suction seemed to be working and we'd get a lovely swirling going on, but it wouldn't drain, and we seemed to have accomplished nothing at all and abandoned it. A little while later, the water level was higher than it had been.

What's going on and what should we do? I'm game to get under the sink with a large wrench and bucket and see if something's in that bendy bit under there, but my fiance's not so keen to jump into it...he's nervous that the refilling indicates a worse problem than just a clog.

Thanks so much for any help!!

water

It indicates that something is connected to the sink drain and is sending the water into it. The most likely thing would be an air conditioner's condensate drain. Clearing the drain will cure all the problems, but chemical drain cleaners are not the way to do it, neither is removing the trap because the stoppage is normally further down the line than that.

Thanks, hj! Does it sound like it's something we can do ourselves with a snake?

Sounds like either something like the condensate or possibly a clog downstream of the lav after another fixture connects to the drain...
This fixture is then causing the lav to flood...
Is there any fixture near to the lav that, when used, seems to cause the problem to get worse?
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Taking the PTrap apart is not hard - it will be stinky as it sounds like you have a nasty slimy mess there - get some rubber gloves, a wrench/pair of channel lock pliers, and a bucket and remove the trap...
If the trap is clogged, replace it or clean it out...(probably not the trap)
If it is not the trap, run a snake down the drain pipe that the trap was connected to - the leak is probably fairly close to the lav( within 15' or so...) or else more fixtures would be affected...

Since this is upstairs, there normally aren't sources of water that get into the drains to cause a sink to back up...a shower maybe. Normally, it is higher than the other sources of water.

The likely sources are as indicated - either an a/c condensate, or maybe overflow from a humidifier (is the furnace/humidifier or a/c evaporator in the attic or on the roof?).

Another possiblitiy is if it has been raining a lot, but it would take a bunch to back up a sink. As noted, there must be a blockage, and it is probably not too far away, as it if was, the toilet and other things would be backing up, too. If you are lucky, after you take the trap off, you'll find it close by.