Ok like @oldturkey03 said it's probably battery and if lithium is drained below 2.7v most systems will not attempt to charge it. Sometime you can successfully being it back to life with a 18650 battery. Only have it connected in small quick burst while the unit is unplugged. They will heat up pretty fast so 5 seconds bursts. Then let it cool. Soon as you get it to >=2.8v remove 18650 and try charging normally.

If not keeping or taking any charge at all than take a picture of the battery pack & I'll find you a link for a replacement