My iPhone 6S doesn't turn on, doesn't let me hard resetit neither

It was working perfectly today but suddenly, it was like really slow. I was typing the unlock pw and it was really slow and it was alreay in 2% battery, when I saw that, I immediately charged it. I had it connected for around and hour and it was still in the same 2% battery. I unplugged it to let it turn off and recharge it "from scratch" and it never turned on again.

I googled same issue and I found that I should:

1. Connect the device to your computer

2. Shut down the device

3. Press the Power and the Home buttons at the same time

4. After 10 seconds release the Power button but continue holding the Home button

until you see a popup from iTunes saying "iTunes has detected an iPhone in recovery mode"

5. If you have been successful your device screen will be completely black/blank (this is 'DFU Mode')

6. You can now proceed and restore to the latest firmware using iTunes

But nothing worked. I found other procedures but nothing worked. My phone is not even recognized by iTunes nor my laptop (windows) itself.

As well, when the phone is not connected to the laptop, the screen is completely black, but when I connect it, the apple logo appears for approximately 2 seconds and it turns off (black screen) for like other 3-4 seconds and then the logo again for 2 seconds and the same pattern continues.

I feel really really bad, because I just bought the phone (used) and it is not longer under warranty. Has not been dropped and I have taken good care of it and now, this is happening.

Please help. What could be happening?