It's either gotten under the lockout voltage or it's bad. Try letting it sit overnight charging in the controller first. If that doesn't work, then take a bench power supply and force 4v in there for a coupe of minutes, keeping an eye on the battery the whole time. Remove the power and check to see if it's over 3.5v. If it is, then toss it in the controller again and let it charge, if it is still under 3v then recycle it and get a new on.