Software update: stuck in DFU mode

I want everyone to see the situation clear, so I should start a bit earlier: The story starts when I turned off my phone in October and I could not turn it on again: went to a Premium Reseller’s service, got a new battery (it was the problem), the problem has been solved…or at least I thought so. A bit later, I started a software update, which has not started, instead of that, the phone stuck in DFU mode. I tried hard reset many times…Some hours later, hard reset worked, software update started, life went on. That happened twice, today is the third time: my phone started a software update during the night while I was charging it, and now it is not turning on. Maybe it will turn on later that day, but I wonder what the base problem can be. Do you have any ideas? I know that I should take it to an Apple service, but before that, I would like to hear some other experiences or possible solutions. Thank you in advance!