I've tried to charge my battery which I didn't use for a while (~2-3 weeks) and it won't charge
While pressing on power button LEDs not blinking, is anybody facing the same issue? How to fix it?

Are you sure that the battery is not in hibernation mode? If a battery is depleted and stored for a long time, it goes into hibernation mode to protect itself. If that happens, you have to plug it in and just let it sit for awhile. There is a DJI manual for the batteries HERE (look under the "Battery Storage" section) that might be helpful. I have read some posts that say that you have to leave the battery in the charger for up to an hour before it comes out of hibernation mode.

Hi Mikex, my sincere apologies for the inconvenience that it caused. Kindly check  the battery terminals pins and make sure it is clean and nothing stuck on it. For the hibernation mode, the battery will automatically turn off when the there is no current for 20 minutes after turning on. Also, when the voltage is lower than 2.75 v or the battery level is lower than 10% and the battery has been stored for more than 6 hours. Do you have a different battery that you can try so that we can isolate the issue? Please provide a short video of the issue for further checking. You can upload it by using google-drive or Dropbox and provide the link here. Thank you for your continued support.