Hardening off needs to be a very gradual process. Brief periods of time at first without direct sun or wind exposure.

Without seeing them or at least a photo of them I can only hazard the guess that it is damp-off, a common seedling fungal disease, with perhaps some stem wind damage thrown in.

If you Google 'damping-off' you'll find many photos of it to compare to your plants. There is also a FAQ here about it that I linked below but it has no photos.

Re-potting and burying the damage part of the stem should save some of them unless the damage is too great.

Dave

Here is a link that might be useful: Damp-off FAQ

It sure sounds like damping off to me as well.

Most folks recognize it in newly emerged seedlings but it can affect seedlings up to maybe 4-6 inches tall as well.

Dave, I don't think repotting and burying the stem deeply can help b'c the plants are infected throughout and it just took time for the fungi to catch up with growth and show the first symptoms,

That being said, what's to lose at this point, as in why not try repotting and see what happens.

Carolyn