I cannot turn on my iPhone 5s. Please HELP!!

All of a sudden tonight, my iPhone 5s restarted, but it only got to the apple logo screen, then restarted again. It was doing that repeatedly every 10 seconds or so for the last few hours. The only thing I could do was turn it off completely but only if it wasn't plugged in. If I plugged it into a power source it kept restarting and I couldn't even turn it off. But turning it off didn't do anything anyway because when I turned it on again it just continued doing the same restart cycle. I tried a reset, which had no effect at all. I tried to do a complete restore using itunes, several times.

So I don't know what options I have left. As far as I can tell, I have a brick that lights up with an apple logo or itunes logo.

One thing that did happen earlier today that could be related: I tried to update my iphone (I believe I have 10.3.2 and it was updating to 10.3.3?) but something weird happened during the update (can't remember exactly) and I ended up having to restart my phone without it finishing the update. It wouldn't let me update after that, which I didn't really care about, so I just left it with the ios version I had. Then later in the evening this happened.

One other small thing to note (probably insignificant)-- When the phone kept going through the apple logo loop, right before the screen went black and it restarted, the apple logo would grow dim and there was sometimes a horizontal line across the screen, then it went to black.

If anybody has any ideas, please help! It would be much appreciated. I don't want to have to buy a new phone so soon. Apple products are supposed to have a long life span! And I've never jailbroken it or done anything out of the ordinary.