you have been eaten away. get urself a big fat straw, slit it down the middle , place around the stem. 2/3 inches is all u need, and plant gently in soil....
it rescued mine once, no guarantee, but worth a shot
Once initiated, damping off is fatal! Sometimes called Pythium wilt, is often found in soil and growing mediums.Will also rot older plants at soil line. At onset, the stem loses girth at soil line weakens, grows dark, fluid circulation ceases, plant dies.