Had something similar happen to me. I started my first grow which im still in (3 weeks deep now). And a couple of my plants did that to me. I did some research and discovered there is a plant disease out there. That kill's the roots so that your plant can't drink or eat. It comes from not sterlizing everything you give your plant doesnt affect many weed plant's but sometimes it's seen. You have to try to sterlize everything you put in that dirt. Don't use tap water. I mean you can use it but boil it let get room temp then use it. I usally keep 4 gallon's under my kitchen sink. But seriously I kept my babies in the dirt tring to flush them changing the dirt. And it seemed hopeless so i dumped the couple that had did that. You can't reuse the dirt as well not a good idea. Even if the plant does make it don't reuse the dirt what keeps one plant alive can make another plant die. There is a plant disease solution out there. That you might want to try. But you have to use it once a week for about 5 weeks straight to see results. And it is said to affect taste and quality of the weed. So unless those are the only seeds you got i don't recommend doing that. Best bet is to start over with fresh seed's disinfect the pots before you reuse them. But if you try something and it works let us know!!! Keep hope alive brother