How good is your lighting? If you have sufficient lighting, you can just leave it sitting on the floor of your tank and it will manage to survive and maybe even grow some new leaves. Also, make sure you didn't bury the rhizome.
i have the same kind of problem i have mine on drist wood and also have a mat the grow new plants on leaves but not that nice green color always getting black spots i tried everything except adding potassium and phosphurus. always get black dots on leaves then start turning brown its only my javas to my swords do well in my tank except for the algea problem i have right now same with my crypts for some reason ferns are only ones that get that brown on them and then dye off slowly cant find what problem is i tryed everything people tould me to do. there has only been 1 java fern i had success with and i literally went through hell with it i baught it off the internet said it came from florida 2 weeks notice comes in mail saying i have a package i need to sign for and fill out paper work for and pick up at the main usps shipping center in my area wich was the fern it came from hondour china i think or were ever honour is is said on package hondour malaysia replica of china or something like that that was only fern i baught that stayed true green and did anything then i sold it. i never have baught from hobbyiests though so could be different story but every us website i baught from all junk leaves turned borown and blakc and died one from china never got black always was green and had ne wplants on leaves growing every day but got to big so i sold itit was a mat