This happened to me today. I was not using the phone and had just set it on the bedside table and left it untouched, fully charged, for about 2 hours. I then picked it up to use it and weirdly, the screen wouldn't come on and I couldn't turn the phone on. I had not powered it down. Tried the up volume, down volume, hold lock button many times and it wouldn't come on. My phone literally died completely out of nowhere. Luckily I live about 15 minutes from an Apple store so I went there. The tech plugged it up to their device to check it at the store, which pretty much verified my phone was dead as a doornail. I purchased this brand new iPhone XS Max on approximately December 14, 2018, and it is just now three months old on March 17, 2019 (needless to say, made for a very "unlucky" St. Patrick's Day). However Apple said I had a year warranty and they replaced it with a new phone no questions asked and helped me set the new phone up.
Overall it stunk that my phone mysteriously died, but I was very pleased with how the Apple store resolved my problem. Thankfully it happened now, and on a weekend, rather than in 2 weeks when I will be traveling out of state all by myself and my phone would've literally been my lifeline for communication and directions to get to and from home on my trip!
The tech also mentioned that my messed up phone would be sent away for Apple to check what happened and make sure it hopefully doesn't happen to new phones in the future. However, I will never actually know what caused my phone to mysteriously and suddenly die. It had never so much as glitched before today. Also served as a reminder for me to ALWAYS back up my data no matter how careful I am with the device! Hope you can get your issue resolved in a similar way if your phone will not come back on.