Ok, im not going to tell you that this is a "cure" or how it should be done. And, there will be members who are going to discredit me, and may even flame me for this, but I have done it more then once, and it has worked for me. So you can take my advise, or move on.
Roots continue to grow during darkness. Yes, photsynthesis comes to a halt, but the roots continue growing.
On occasion, i will have a cloneor two that just end up stalling, and doin nothing. The first time tgis happened to me, I had my clones in rapid rooters, and after almost 20 days of them doing nothing, i threw the clones and rapid rooters in the garbage. 3 or 4 days went by, and I was cleaning up my grow area. When I went to throw out garbage, there were the rapid rooters, in the garbage bag, with roots every where!!! I grabbed em out and threw em in soil. Plants ended up being beautiful and very healthy.
Anyways, since then, when ever I have a clone stall out on me, I will throw them into complete darkness for 3-5 days, checking them once a day, and they always rebound and beguin to show life again. I do not know why, but it works.