It happened with my Phantom 3, I found a used one on ebay quite reasonable (he had crashed the drone) but later got mine charging again by opening the case and checking connections to the battery.
Look at some youtube guides before opening it if you go that route, there is a fiddly (but quite large) ribbon cable has to be unclipped.

If it suddenly stopped taking charge at all rather than wouldn't hold power like before, I suspect it is a connection issue rather than failed battery.