I'd slip the rootball out, and I'd cut-off the rotted roots with a sharp pair of scissors, and re-pot in fresh soil. Raise lights for the first couple of days, but keep 'em on the same schedule. No nutes till new foliar growth appears.

Might need stakes or supports till it fills-in your net pots or whatever you hydro guys use to hold the roots.

Cannabis is pretty resilient, so will likely come through just fine. I do this on purpose all the time. (albeit in soil) Click on my Bonsai link in my signature. Same basic technique. :thumbsup: