Need to make a correction. This is not a Nyagous, but instead is a Jaunne Flamme. The other tomatoes around it are also Jaunne Flammes, all from the same source.

Is that first leaf damaged? Is that possible physical damage to the stem I see near it in that first pic? Examine the main stem closely. Move aside the mulch and check the base of the plant for any damage. If you can without damaging roots, push aside a little of the dirt right next to the base so you can examine the stem that is just under the soil as well.

Second, check the soil a couple of inches down under the surface in a coupld places near the plant to confirm correct moisture levels on par with the rest of the bed.

Fusarium and verticilium normally don't look like that, and the wilting part is much slower and not usually the entire plant at once. And they aren't an emergency that requires immediate pulling of the plant, anyway. Once the organism is in your soil, it's in your soil, but it spreads slowly, so you'd have a little time to determine the cause.

It could be bacterial wilt, which makes the entire plant wilt quickly at once without much yellowing and leaf symptoms, but it's only June and you're in 5b, so that's pretty unlikely.

My guesses are:
1) there's some physical damage to the base of the plant compromising the plant's circulatory system, or possibly some kind of rot at the base under the mulch
2) your hugelbed has some kind of pocket in it that may be causing drainage issues, either too much or too little
3) you may have a stalk borer. See if you notice any bore holes as you examine the plant.

I would start with those possibilities.

Yes, you are correct. Checking underneath,

Under the soil line I found a severe break. I had also tested for bacterial wilt, and it was negative. I should have looked for that initially, but I must admit I panicked, as I hadn't looked at the plants in some days. Thanks!

Yeah, it looked like the kind of wilt that happened from physical damage. Disease doesn't look like that. Even bacterial wilt usually tends to wilt the leaves first before the stems start to collapse. Sorry you lost your plant, but at least you don't have to worry about disease that may harm your other plants.

Jaune Flammé is indeterminate, so if you cut your plant back a lot to reduce its water needs but a couple nodes and pile up some dirt around the damaged stem area, it may survive and grow additional roots and come back and give you a few fruit, as long as the stem is still at least partially attached. Or if you have any early varieties among your other plants, you could try rooting a sucker and planting it in that spot. It'll be late, but it may give you some fruit.

Ideally, you would be able to buy a replacement tomato plant to fill the space, but it may be too late to find any for sale at this point.

I probably could find another good heirloom at the farmers market here this weekend. Around here, most people aren't putting in tomato plants til the end of May. Temperatures were in the low 40's here the week before Memorial Day. I do have 5 Jaune Flammes left, so I'm not too fussed. Our season is on the short side, so I don't think the plant would give me much after nursing it back to health. I do have a couple of cukes that need a new home, so I might just put them there instead. Thank you all for your advice!