first, the fact that you replaced an outlet that was on an apparently unknown circuit underscores the importance of mapping all of the circuits in your house and always testing hot and neutral before touching them. if it had been a faulty outlet and you killed the power to the wrong circuit, it would have not been pleasant when you tried to replace the outlet. the first thing that came to mind with your description is that the one functioning outlet is the last in the series and the wires on the "load" side (so to speak, even though it isn't GFCI) somehow disconnected or the outlet itself is bad. that means everything on the other side of it won't work. just a thought. please don't touch any wires where there is a remote possibility that they are live. always test first.

The NEC is the National Electric Code, but jurisdictions can adopt it in while, in part, or change any part they want for local use (depending on what authority they have, especially in Dillon Rule states). The small appliance branch circuits under the NEC are restricted to the wall-counter space receptacles in the kitchen, and with just a few exceptions (gas range, refrigerator if you really want to (IIRC)) but NO other outlets are allowed. No garbage disposals, dishwashers, lights, receptacles NOT on the wall-counter space. It is easy enough to overload the circuits with counter appliances that ARE allowed without allowing them to go anywhere else. Coffee makers, toasters, toaster ovens, and microwaves ca overload ONE of the circuits easily enough to cause nuisance tripping. By making sure appliances commonly used together are on the two separate circuits a lot of nuisance tripping can be eliminated. It can still be a problem if you try to use the microwave with a large kitchen mixer on the same circuit. If the two appliances are both started at the same time it may trip (both pull more power ant initial start up). Many kitchen appliances are very close to 15 amps (1800 watts) and at least part of the time may be over that briefly. Put two of them on the same 20 amp circuit and a nuisance trip is possible. The fact that many also cycle on and off in operation (microwave on less than 100% output, coffee makers pot heater, taster oven on temperature setting below full blast) and you may go weeks with little problem, and then have it happen almost daily, and then even stop again. If you have a lot of higher power kitchen appliances that may get used together you could even consider more than the minimum required two circuits.

Thanks for the reply, Brickeyee, and I'm sorry I couldn't get back to this sooner. I've read a bit about the problems with harmonics that seem to happen mostly in commercial settings where a bunch of computers or fluorescent lighting is wired on MWBCs on three-phase Wye systems, and the arguments for so-called super-neutrals. But what I suppose I'm not getting in the context of the OP's scenario is the effect on the neutral when the currents are 180 degrees apart. I would have thought that the neutral would still present a decent enough in-phase 0V reference to the computer's power supply when it's fed by an opposite-pole leg. But if it's inductance that really tends to confound things, that's beginning to make sense to me. terribletom - Just trying to understand the angle you are coming from. Is your argument that the neutrals are going to be connected in the panel anyway? Thanks for asking, billl. Well, no, I wasn't theorizing to that extent. I was merely seeking to isolate the problem beyond mere voltage drop (which can, of course, be "felt" by independent ccts from the same panel--more so if it's a small panel and the two hots come from the same pole). It's specifically the current-carrying characteristics of a shared neutral on a MWBC that I was interested in. So I thought that by distinguishing between circuit types, I'd get a more targeted answer. Let's say this is more about learning than arguing, eh?

leela4 and Jan Moyer - Leela4 thank you for your comment and suggestion. We renovated and expanded a small dark kitchen - and the only way to do it in this house (without pretty much gutting my entire first floor to move rooms around) was to make it open concept. We had a kitchen designer. We looked at a model on paper etc etc. The kitchen itself is really beautiful. The space it opened up to is my concern. It was hard to envision how it would look even with the plans and 3D renderings etc. Once the cabinets and breakfast bar went in I realized the separation to the family room wasn’t as much as I envisioned it would. Too late to change at that point. Then I thought I would close off the smaller room next to the kitchen dining area ... but realized last week it would make the dining area seem tight. We had the best of intentions but it didn’t turn out 100 pct as I had hoped. Again, don’t see how we could have done anything differently with the space we had. I have no choice but to work with what I have and will bring in some designers to consult and ultimately pick one to help me tie the spaces together as best I can. Looking back maybe having a designer on board in addition to the kitchen designer up front would have made sense ... but what’s done is done. I don’t appreciate anyone pointing out the negatives of an open kitchen. That doesn’t help. Aren’t comments on Houzz supposed to be helpful when someone is looking for advice? If you can’t help, don’t comment is my feeling.