Question
how can i rehydrate a stressed orchid with rotted roots and without them, but nothing of roots;all roots are black, rotted and moshy.Or what can I do to save it

Answer
Jorge, on an orchid with rotted roots, the roots are not necessarily black.  Orchid roots have an outer sheath called a velamin. When rot sets in, this outer sheath collapses.  This makes a rotted root "squishy" and lack substance.  An orchid root that is firm (regardless of color)is healthy. The procedure to "save" an orchid depends upon the type of orchid. It also depends upon the condition of the rest of the plant.  If you are referring to a phalaenopsis orchid that has no leaves or roots likely cannot be rescued.