It sounds like damping off, a fungal disease. It happens when seedlings are warm and wet. Best things to do: get a fan for air flow and have it gently blow on the seedlings. Also water plants from the bottom not from the top. I also use a fungal spray (Safer 3 in 1) on my seedlings. Use it sparingly on seedlings. You don't want to overwhelm the seedlings with a fungicide.

Agree it is damp-off. Usually only thrives when the soil is being consistently kept too wet and there is lack of air circulation.

You'll find a good FAQ on Damping-off with all the details over on the Growing from Seed forum here.

Dave

Here is a link that might be useful: Growing from Seed forum

THANK YOU Becky and Dave JOHN

You didn't say how big these plants were. Damping off happens when they are small seedlings, an inch or 2, less commonly when they are bigger.

If these are more mature plants past the seedling stage, it could be due to overwatering or overfertilizing or not transplanting before they get rootbound.