The battery itself is likely still good. But it must be "jump started". Sometimes, (but seldom), it will spontaneously start charging again. There are also several possible, but difficult, methods of restarting it. You can prevent this in the future by NEVER allowing your PC or cell battery to come close to completely discharging. First, a little background:
Inside your battery pack, there is a master program circuit that is designed so that if it does allow charging, it doesn't explode from overheating. These programmed circuits simply determine if each individual cell inside is already charged to at least about 10%. If it did allow charging when a cell is completely dead, it would take much more power to charge, which creates a lot more heat. Overheating is physically dangerous to each cell, the battery pack, your PC, you, and those around you, since cells do occasionally explode.
Inside each individual cell, there's also another circuit that looks at its own charged state to determine if it will allow or disallow charging. Your battery pack and/or an individual cell is is "off" right now.
Depending on the circuitry of your individual battery pack, there are ways of quickly overcharging without explosion. There are expensive devices that reportedly can bring back about 40% of the "totally dead" cell batteries. Without this overcharging device, your only option is to open the battery pack and/or an individual cell, bypass the safely circuitry, and quickly charge it without overheating it. As soon as it is charged above that safety threshold, your battery pack will be fine once again. Just find someone that has enough discrete electrical component engineering experience before you try this.
I've written more about PC batteries here: What happens when laptops reach 100% charge?
[EDITS added]
For a more thorough, authoritative, and better understanding, this is a great jumping off point to perhaps find a way to revive, (turn back on), your dead battery: http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries
On their site is also an extremely helpful page about battery pack repair. http://batteryuniversity.com/learn/article/how_to_repair_a_laptop_battery I was shocked to learn just how smart these packs have become. Even more disturbing is mention of SUPER-secret OEM codes they won't reveal, that are sometimes required to turn batteries back on after x amount of use! (I'd like to expose any OEMs' products that employ this despicable practice.) I also found this extremely hopeful comment from "BatteryHacker" on this page from someone that discovered how they easily fixed their dead battery:
This Dell NDE076 replacement battery has been replaced in warranty 3 times in 1.5 years!!! Because somehow after a time, its controller circuit gave up. That means no charging, no energy, no charge report anymore. After the 3rd replacement, I decided to open it and saw the controller (Atmel mega406). The cells were charged, somehow the controller went in a state (probably blocked), and that I had to reset it. It has a RESET pin :)) ! That helped, and [then] I had to fully charge and discharge to teach the controller again. I think I will have to repeat it if the problem occurs again!
[Evidently] One must know there is firmware in this controller and this “blocked state” issue can occur due to a firmware/hardware bug.
