Nikon d3100 battery suddenly won't charge?

I recently borrowed my friend's Nikon d3100. He was kind of iffy about letting me borrow it, but I finally convinced him to. I was using the camera fine and not mishandling or mistreating it in anyway. I used the camera until it said that the battery was too low to keep taking videos, so I turned it off, and went back to my house. I took the battery pack out and put it in the wall charger. I let it charge overnight and then the next day I went to get it of the charger, but then I noticed that the 'still charging' light was still blinking. I even let it charge all day today, but when I tried to use it again, it still said that the battery was too low to do anything and that I needed to charge it. So now I might have to buy my friend a new battery pack because it just crapped out on me out of nowhere. Is there anything that might have caused this, or a way to fix it maybe? I would appreciate any feedback because I really don't want to have to buy my friend, who barely trusted me with his camera, a new battery pack.

Sometimes, on rare occasions, when you plug in a battery into the battery pack, it does not make good contact, and the battery will not charge. If this happens, simple remove the battery from the charger and reinsert it in again.

So try that.

When you stated you turned the camera off when the battery was low, did you do it immediately? Sometimes people will get a battery low indication, but try to eek out a few more photos or video. Then the camera turns off. So they turn the camera back on, and take a few more photos until the camera turns off again. And they might repeat this a couple of times.

This is the surest way to damage a battery, and if (and I say if) you did that - you owe your friend a new battery.

However, since your friend let you use his camera, you should buy him a new battery or some other gift regardless if you damaged the battery or not - just for letting you use the camera.

You used battery charger whole day and got no advantage, I guess the problem is with the battery or got to check with some options in cam…

Now you know why your friend was "iffy" about it. DSLR's to me are like underwear. You don't lend it to anyone else. That's why I bought my wife and two kids dSLR's of their own.

First, try the battery out. Sometimes, when fully charged, the status in the LCD comes up empty. Fire a shot with flash. If it worked. Turn off the camera then turn on again. The battery status should come out already. If it did not work, Clean the electrical contacts of both battery and charger then try to recharge again. If the battery fails to charge then bring both charger and battery to the store and let them check which one you should be buying.

While at the store, it would be a good idea to buy your own camera now also.