Hello Myztaix. If your DS had not been turn on in years the battery is probaby in deep-discharge. This means the voltage is so low, your DS believes there is no battery installed. If it believes there is no battery, it will not attempt to charge the dead on that is installed. This same situation can happen to any and all portable electronics. Often the original battery can be recovered. First you need to remove the battery and find a universal battery charger suitable for 3.7v batteries. Many cellphone shops or camera shops will sell them for $5-15. A universal battery charger will clip to your battery and *probably* get it out of it's deep-discharge state. They tend to be slow, taking hours to fully charge a cell phone sized battery. Alternatively if you have a regulated DC power supply, you can set it to 4.5 or 5Volts and directly attach wires from your power supply to the battery terminals. Note that this method is *potentially very dangerous*. I would only use this option if the battery terminals are clearly marked as positive and negative. Also I would only charge for a maximum of 5 minutes between checking the temperature of the battery by hand. Anything above room temperature is bad - stop and wait for it too cool before you continue.
Now alternativly, if you just buy a new battery it should arrive half-charged and you can avoid all the work. In the future charge any battery to around 50% before storing it for long periods. Lithium ion batteries are happy in the middle- do not sure or ship them dead, do not store or ship them full. If you can, take the battery out of a device before storing.