if it looks like this, it's Pythium Wilt (damping off) ... usually results from too-wet environment, frequently from pre-soaking seeds, and the wet paper-towel thing ... (Mother Nature doesn't do it that way, why do some of us ?) ... ... I have tried swabbing the stems, with H2O2, in an effort to stop the fungal growth, didn't seem to help ... let it dry out, as much as you can, but don't get too hopeful ... there may be a simple fix to this, but I don't know of it ... in this photo, this was the only seedling to suffer from it, in a group of 13 ... it's a true plant-killer