Crown rot causes the plants to decay at the soil line.
The disease is caused by Pellicularis rolfsii and no
chemical control is listed. Stem rot can be caused by several different fungi.

I'm having a similar issue (slight wilt rather than very), with about the same timing. I just assumed it was due to the unusual weather this year. Most of my plants are xeric, so a few have been very unhappy with having so much water.

Thanks, both. We're also thinking maybe voles; we found a hole about 8 feet away a few months ago. Still investigating, and the plants are looking like they're on their last legs.

Susanka, If it really looks like they're going to die, do any of the stems seem to be in good enough shape yet that you could make some cuttings to start some new ones? I recommend making some tip cutings, and some stem cuttings. I've had trouble rooting tip cuttings from the upright sedums, and I'm thinking the stem cuttings might do better--but I'm not positive about that! The other possibility would be to dig them up and put the best looking rooted pieces into pots to get them going again. That way you can control the water---and be sure there aren't any critters getting at them!

Good luck,
Skybird

Thank you, Skybird. I didn't know they'd start from cuttings. We're planning to investigate further as to whether it's voles, but they really look like they're on their last legs (the sedum, not the voles). Thanks for the tip about cuttings.