something keeps cutting my plants between the roots and stem.+rep

Wasup RIU It has been a while since I have posted but I need your help.

Two of my biggest outdoors plants have been killed in the last 3 days by something. I have dug all through the soil looking for what it could of been but found nothing. The plant is just laid over with the roots completely cut from the stem, right at my medium level, please help because whatever it is, it is taking out a plant every couple of days. Oh yea another question, I need to know if I can bring back my plant that has 1 root coming of still, it was the only one that wasn't cut completely. If so what can I do to increase its chance of survival. I planted it deeper and watered really good with very light dosage of Dutchmasters Grow A n B.

come on guys, rollitup is filled with very educated people, I know that at least one of you one here has had this problem with outdoor plants before. I cant be the first, hopefully because if so I am fucked.lol I dont really stress when it comes to plants dying because shit happens and you can always grow more but I do want to save these because it is already so late in the season. The plants are around 3ft tall dark green, and perfectly healthy it sucks they keep dying over night.

Ihad the same prob but mine was termites they will destroy a plant in 24hrs. Also i had a 6ftr laid over and i stood her back up and staked her off AND water her good and now she is in full flower