I've had the Gear Fit Pro 2 for maybe 3 or 4 years with almost no problems. About a week or so ago, I left the watch run without being on the charger. Later when I wanted to use it, it was completely dead. Tried to charge it for a couple of days without success. I would get a momentary "lightning bolt" and then it would disappear.
I was about to toss it, but thought I would give it one more trip. Back on the charger, but this time I held it very firmly in the charger position and the weak lightning bolt remained dimly on. After about 10 minutes I released the pressure that I was applying and the lightning bolt stayed on. I let it continue to charge for about 30 more minutes and it was either 100% charged or close to it.
Sometimes these rechargeable batteries are hard to get going until they get to at lease 1-2% and then they recharge fine. So instead of my Gear in the waste backet, it's like it was new. So don't give up!