It's very possible that the LiPo has been over-discharged to the point that its voltage is now below what the RC charging circuits expects, so it doesn't even try to charge it. If the battery still has a "pulse" (meaning: does not read 0V) and you know what you're doing (!), applying the right DC voltage directly to the battery outer leads for 15 minutes or so can bring it up to a level where it gets properly detected by the RC, which will take it from there. I'm well aware that a LiPo could be damaged beyond repair by over-discharging it, but I also have direct experience that what I just explained can work (it happened to me a few years ago with a 3S LiPo pack, which is still in use today after being jump-started!).
If your battery is a 2S, something in the 6-7V range should do it, like 4 alkaline AA batteries in series.