Why does my phone shut off for over 15 hours, then reset and say "Abnormal Reset"?

[AC Question]

Sometimes when I'm working on my phone is turns itself off and stops working for up to 15 hours. When it finally resets and turns back on it says "Abnormal Reset." I have the HTC desire 626s and have only had it 9 days. I'm not sure what the problem is, or why it shuts it self down. I cannot take out the battery as that would ruin the phone.