How old are they? If they are very young, a fungus could be causing damping off. Running an oscillating fan, sterile seed-starting mix, adequate drainage, bottom watering, and not overwatering can help prevent this.

Underwatering could also be a cause. Sometimes the soil will dry out very fast under lights and cause the plants to shrivel up.

If the plants are bigger, potting them up to a larger pot may help.

Are they all doing this, or is there any pattern to it? I had the slightest slant in my table that caused one side to get underwatered and another slightly warped container that shorted the plants in the middle.

Do any of the above sound like possibilities? If not, post more details and someone may have an idea.

If they shrivel from the bottom up, and the stem turns dark at that point, from my reading, this sounds like the dreaded "damping off". But if the stems shrivel somewhere towards the top of the stalk, doing so along some length, without any color change, does this suggest the same "problem", or something else?

In my case it's cabbage plants that have shriveled, some at the bottom, and now a few days later, a few others shriveling towards the top, just below the leaves.

But others planted at the same time and in pretty much the exact same conditions, are doing just fine so far.

Thanks,
Jim